GRE Sample Test 2 Verbal Reasoning 2

What sort of atmosphere would make Mars the only planet “whose surface details can be discerned from Earth?” You need a word that means transparent or thin for the blank. Viscous takes you in the wrong direction, so toss it. The next choice, ossified, makes no sense; toss that one too. In contrast, rarefied works well, so hang onto it.
Meanwhile, a copious atmosphere would definitely not be easy to see through, so cross out that choice. Meager fits nicely and agrees with rarefied, making those two the correct answers.