A. The GRE wants you to assume that since there are more males than females among the students, that there must be more males studying the physical sciences as well. But since it's entirely possible that every student studying physical sciences is female, statement A doesn't have to be true.
B. Since social sciences is 30% and physical sciences is 24%, they total 54%, which is greater than half of the total enrollment, so statement B must be true.
C. The number of senior males is 160, while the number of senior females is 84, producing a 160:84 ratio. A ratio of 160:80 would of course be 2:1, but since in the actual ratio, 84 is more than 80, we will get a ratio that is less than 2:1. Thus statement C must be true.
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