ACT English & Reading · Punctuation: commas
Which choice makes the sentence most grammatically acceptable?
A Heart That Still Moves
When New Horizons flew past Pluto in July of 2015, [1] one feature drew more attention than any other: a pale, heart-shaped region roughly the size of Texas, its left lobe a smooth basin unlike anything else on the dwarf planet's surface. That basin, called Sputnik Planitia, [2] is a glacier made almost entirely of frozen nitrogen, and its surface tells a story of ice that has never stopped moving.
Close images revealed a pattern of polygon shaped cells across the glacier's northern half [3], each one tens of kilometers wide with a low ridge at its center and a shallow trough along its edges. The pattern is the signature of solid state convection [4]: warmer ice rising slowly at the center of each cell, cooling near the surface, and sinking again at the edges, a process that continually resurfaces the glacier from within. Scientists estimate the surface is quite young, [5] probably less than a million years old, because the slow churning erases craters almost as quickly as impacts could create them.
The glacier is not static at its edges either. [6] East of Sputnik Planitia, smaller nitrogen glaciers descend from the surrounding highlands and empty into the basin the way tributary glaciers feed a larger ice sheet on Earth. [7] Unlike Earth's water ice glaciers, however, any melting inside a nitrogen glacier behaves in reverse: liquid nitrogen is less dense than the solid ice around it, [8] so it rises toward the surface instead of sinking toward the bottom.
A 2026 analysis of the original 2015 imagery added a further twist. [9] Researchers identified dark streaks and patches along the glacier's northern edge that closely resemble features on Earth's ice sheets where liquid water has recently wetted the surface. Because sunlight and other familiar heat sources are far too weak to explain such sharply defined dark patterns on Pluto, [10] the most likely explanation is liquid nitrogen reaching the surface from below, through a process of melting at the glacier's base rather than from above.
If the interpretation holds, Sputnik Planitia would be one of only a handful of places in the solar system, alongside Earth and a few icy moons, [11] where liquid currently moves across a planetary surface, more than nine billion kilometers from the sun that most such activity elsewhere depends on.
Close images revealed a pattern of polygon shaped cells across the glacier's northern half [3], each one tens of kilometers wide with a low ridge at its center and a shallow trough along its edges. The pattern is the signature of solid state convection [4]: warmer ice rising slowly at the center of each cell, cooling near the surface, and sinking again at the edges, a process that continually resurfaces the glacier from within. Scientists estimate the surface is quite young, [5] probably less than a million years old, because the slow churning erases craters almost as quickly as impacts could create them.
The glacier is not static at its edges either. [6] East of Sputnik Planitia, smaller nitrogen glaciers descend from the surrounding highlands and empty into the basin the way tributary glaciers feed a larger ice sheet on Earth. [7] Unlike Earth's water ice glaciers, however, any melting inside a nitrogen glacier behaves in reverse: liquid nitrogen is less dense than the solid ice around it, [8] so it rises toward the surface instead of sinking toward the bottom.
A 2026 analysis of the original 2015 imagery added a further twist. [9] Researchers identified dark streaks and patches along the glacier's northern edge that closely resemble features on Earth's ice sheets where liquid water has recently wetted the surface. Because sunlight and other familiar heat sources are far too weak to explain such sharply defined dark patterns on Pluto, [10] the most likely explanation is liquid nitrogen reaching the surface from below, through a process of melting at the glacier's base rather than from above.
If the interpretation holds, Sputnik Planitia would be one of only a handful of places in the solar system, alongside Earth and a few icy moons, [11] where liquid currently moves across a planetary surface, more than nine billion kilometers from the sun that most such activity elsewhere depends on.
Which choice makes the sentence most grammatically acceptable?
Answer and explanation
Answer: D: Because sunlight and other familiar heat sources are far too weak
No comma belongs anywhere in this introductory clause, since "sunlight and other familiar heat sources" is a single compound subject and "are far too weak" is its necessary verb phrase, with the clause's one needed comma correctly placed later, after "Pluto." Choice C incorrectly separates the compound subject from its verb. A incorrectly splits the compound subject itself with an unneeded comma. B, the original, contains the same error as C.
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