OG1 阅读 Q22

Choice B is the best answer. (Lines 2-4"DNA...groups") of the passage describe DNA as "a very long chain, the backbone of which consists of a regular alternation of sugar and phosphate groups." The backbone of DNA, in other words, is the main structure of a chain made up of repeating units of sugar and phosphate.
Choice A is incorrect because the passage describes DNA on the molecular level only and never mentions the spinal column of organisms. Choice C is incorrect because the passage describes the backbone of the molecule as having "a regular alternation" of sugar and phosphate, not one or the other. Choice D is incorrect because the nitrogenous bases are not the main struc-tural unit of DNA; rather, they are attached only to the repeating units of sugar.