(A) lends support to the idea that our ancestors did interbreed with Neandaerthals, not that they didn’t. Answer choices that weaken our argument are never necessary, so (A) is not our answer.

(B) weakens our argument by suggesting the DNA tests might be unreliable. If we can’t trust the accuracy of the tests, how could we draw accurate conclusions based on their results?

If (C) is not true—if modern human DNA is very different from that of both Neanderthals and our ancestors—then the argument falls apart. The author’s only evidence is that modern DNA is different from that of Neanderthals, so if it’s different from our ancestors’ DNA as well, the tests provide no evidence they didn’t interbreed.

(D) strengthens our argument, but it’s too strong to be necessary. Even if we negate (D) to say that our ancestors and Neanderthals lived in the same locations, it’s still possible they didn’t interbreed for other reasons.

(E) plays on the assumption that genetic similarities can be a sign of interbreeding, but it is too strong to be necessary. We only need to know that genetic similarities between humans and Neanderthals point to interbreeding, not that all genetic similarities between all species do. If (E) were true, a 1% similarity in DNA between humans and trees would indicate interbreeding, which is not at all necessary for the argument.