ACT English & Reading · Pronoun reference and agreement
Which choice most effectively completes the sentence while maintaining clear pronoun reference?
A Different Kind of Battery
For more than a decade, lithium has dominated battery chemistry so thoroughly that most people simply assumed no serious alternative existed. That assumption is now being tested directly. A different element, sodium, has moved from research curiosity to commercial product with a speed that few engineers in the field predicted even five years ago. [1]
The appeal of sodium starts with simple chemistry. Sodium is roughly a thousand times more abundant in the earth's crust than lithium is, and unlike lithium, it can be extracted from ordinary seawater and common salt deposits rather than from a small number of geographically concentrated mines. Each of the major sodium-ion chemistries currently in development reflects [2] a distinct set of tradeoffs between cost, safety, and how much energy the resulting cell can actually store.
Cost and safety are where sodium's advantage currently shows up most clearly. Sodium-ion cells tend to be cheaper to produce, more stable at high temperatures, and considerably more tolerant of the deep cold that degrades many lithium-ion batteries. What sodium has not yet matched is energy density: a sodium-ion cell generally stores less energy for a given weight than a top lithium-ion cell does, which is precisely why grid-scale storage, where weight matters far less than it does in a moving vehicle, became the technology's first serious commercial market. [3]
That changed, at least partially, in February of 2026. CATL and Changan Automobile jointly released a mass-produced passenger vehicle powered by a sodium-ion battery, a genuine milestone for a technology that most automakers had previously treated as a distant, grid-only prospect. [4] Engineers who had spent years refining the underlying chemistry for stationary storage suddenly found their work adapted for a moving vehicle. It has surprised several industry analysts. [5]
The battery's performance in that first vehicle illustrates both sides of sodium's tradeoff clearly. Its energy density remains below that of a comparable lithium-ion pack, meaning the vehicle's range is somewhat more limited, [6] but its tolerance for extreme temperatures and its comparatively low manufacturing cost makes [7] it genuinely competitive for cost-sensitive, entry-level vehicles rather than for long-range flagship models.
Skeptics of the technology point out, correctly, that sodium-ion batteries represented less than one percent of global battery production as recently as 2025. Between the current pace of investment and the technology's genuine engineering advantages, however, few analysts now dismiss sodium-ion the way many did only a few years earlier. [8] Whether it ever truly rivals lithium in vehicles built for maximum range, or whether it settles permanently into a specific niche of grid storage and affordable transportation, sodium's [9] rapid rise from laboratory curiosity to commercial product is already a completed fact, not a speculative prediction. Manufacturers on multiple continents are scaling production lines. [10]
The appeal of sodium starts with simple chemistry. Sodium is roughly a thousand times more abundant in the earth's crust than lithium is, and unlike lithium, it can be extracted from ordinary seawater and common salt deposits rather than from a small number of geographically concentrated mines. Each of the major sodium-ion chemistries currently in development reflects [2] a distinct set of tradeoffs between cost, safety, and how much energy the resulting cell can actually store.
Cost and safety are where sodium's advantage currently shows up most clearly. Sodium-ion cells tend to be cheaper to produce, more stable at high temperatures, and considerably more tolerant of the deep cold that degrades many lithium-ion batteries. What sodium has not yet matched is energy density: a sodium-ion cell generally stores less energy for a given weight than a top lithium-ion cell does, which is precisely why grid-scale storage, where weight matters far less than it does in a moving vehicle, became the technology's first serious commercial market. [3]
That changed, at least partially, in February of 2026. CATL and Changan Automobile jointly released a mass-produced passenger vehicle powered by a sodium-ion battery, a genuine milestone for a technology that most automakers had previously treated as a distant, grid-only prospect. [4] Engineers who had spent years refining the underlying chemistry for stationary storage suddenly found their work adapted for a moving vehicle. It has surprised several industry analysts. [5]
The battery's performance in that first vehicle illustrates both sides of sodium's tradeoff clearly. Its energy density remains below that of a comparable lithium-ion pack, meaning the vehicle's range is somewhat more limited, [6] but its tolerance for extreme temperatures and its comparatively low manufacturing cost makes [7] it genuinely competitive for cost-sensitive, entry-level vehicles rather than for long-range flagship models.
Skeptics of the technology point out, correctly, that sodium-ion batteries represented less than one percent of global battery production as recently as 2025. Between the current pace of investment and the technology's genuine engineering advantages, however, few analysts now dismiss sodium-ion the way many did only a few years earlier. [8] Whether it ever truly rivals lithium in vehicles built for maximum range, or whether it settles permanently into a specific niche of grid storage and affordable transportation, sodium's [9] rapid rise from laboratory curiosity to commercial product is already a completed fact, not a speculative prediction. Manufacturers on multiple continents are scaling production lines. [10]
Which choice most effectively completes the sentence while maintaining clear pronoun reference?
Answer and explanation
Answer: A: NO CHANGE
The relative pronoun "which," already present in the original, correctly and unambiguously refers back to the entire preceding idea, the density gap just described, without introducing a new, potentially ambiguous pronoun into an already complex sentence. Choice C introduces the plural pronoun "they," which has no clear plural antecedent anywhere in the sentence. B and D each introduce a vague pronoun, "this" or "it," immediately following an already-complex sentence, creating exactly the kind of unclear reference careful editing avoids when a relative clause can do the job more precisely.
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