Q17. The correct answer is indeed C. The passage talks about how civilizations that started at different times would interact, but does not consider that science and technology can improve faster or slower depending on who or what is doing the development (among other factors).
The assumption is that, a civilization which started later than we did would never be able to overtake us, and also that we could never catch up to a civilization more advanced than we are, because all civilizations develop science and technology at roughly the same rate. In reality, we should not assume that all civilizations advance at the same rate. Lines 23 and 24 setup the concept of the "rate of human technology", while lines 26 to 35 are built on this assumption.
Q18. To answer this question, you have to think from the perspective of the author of Passage 2. The author of Passage 2 raises several points, raised by other scientists, that run counter to the claim of "consensus" in Passage 1. It is an oversimplification to say that there is a consensus when in fact there are multiple strong views that are against the possibility that we would discover intelligent extraterrestrial life (or that they would discover us). To say that there is a consensus is to say that since there are no sufficiently strong opinions against X, then the community approves X. But in reality, there isn't a consensus, as there are at least 3 (Fermi, Drake, Zuckerman, and so on) strong, defensible opinions in the community.