What is the distance between the points (2, 3) and (10, 9)?

Use the distance formula: $d=\sqrt{(10-2)^2+(9-3)^2}=\sqrt{64+36}=\sqrt{100}=10$, choice B. The horizontal change is $8$ and the vertical change is $6$, so the segment is the hypotenuse of a 6-8-10 right triangle. Choice A ($8$) is only the horizontal change. Choice C ($14$) adds the two legs, $8+6$, instead of using the Pythagorean theorem. Choice D ($100$) forgets the final square root. A straight segment is always shorter than the sum of the legs of the right triangle it spans, which rules out choice C right away.