I understood the structure of the passage to be: a brief discussion of thesis/antithesis, an example of such a pairing, and a conclusion that goes beyond the initial discussion of thesis/antithesis by explaining the synthesis of cytology and biochemistry.
Based on this, I narrowed the answer choices down to B and C. Each seemed to only cover the first or last 2/3 of the passage; I thought it would be correct to state that there was first a general proposition (in science, a thesis usually comes with an antithesis, and they are related in this way), an example, and a conclusion (they reach a synthesis, and help each other progress -- neither is mentioned in the first paragraph).
The Kaplan explanation says that B is incorrect because it's not a "set" of examples, it's only one example. This just confuses me further because I thought that the cytology/biochemistry example could very well be defined as a set of examples that go together and are both necessary and complementary to the full set of the example (cytology + biochem = molecular genetics)
I would greatly appreciate any insight! Thanks!