OG1 阅读 Q25

Choice C is the best answer. In (lines 12-14"The...around") the authors claim that DNA mol-ecules appear to be comprised of two chains, even though "it has often been assumed . . . there would be only one" (lines 15-17"a...structural"). The authors support this claim with evidence compiled from an X-ray: "the density, taken with the X-ray evidence, suggests very strongly that there are two [chains]" (lines 18-19"unit...two").
Choices A, B, and D are incorrect because the authors mention density and X-ray evidence to support a claim, not to establish that DNA carries genetic information, present a hypothesis about the composition of a nucleotide, or confirm a relationship between the density and chemical formula of DNA.