OE


The blank refers to certain human beings. The clue for the blank is that certain humans may be more… than others because they possess a gene that predisposes them toward aggressive behavior. Therefore, a good word for the blank is “aggressive.” Choices (A), (B), and (C)— reserved, timorous, self-possessed— can be eliminated because they do not match “aggressive.” Choice (D), uncouth, means lacking good manners, which does not match “aggressive,” so eliminate (D). The correct answer is (E), quarrelsome.