OG1 阅读 Q26

Choice B is the best answer. The authors explain that "only certain pairs of bases will fit into the structure" (lines 25-26"base...structure") of the DNA molecule. These pairs must contain "a purine and the other a pyrimidine in order to bridge between the two chains" (lines 27-29"One...for"), which implies that any other pairing would not "fit into the structure" of the DNA molecule. Therefore, a pair of purines would be larger than the required purine/pyrimidine pair and would not fit into the structure of the DNA molecule.
Choice A is incorrect because this section is not discussing the distance between a sugar and phosphate group. Choice C is incorrect because the passage never makes clear the size of the pyrimidines or purines in relation to each other, only in relation to the space needed to bond the chains of the DNA molecule. Choice D is incorrect because the lines do not make an implication about the size of a pair of pyrimidines in relation to the size of a pair consisting of a purine and a pyrimidine.