Our research has focused on a drug which is so as to be able to change brain chemistry. A

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(答案→)A. powerful
B. influential
C. monstrous
D. vigorous
解析：powerfu强有力的。在这里四个形容词中, 只有powerful (有效力的) 可与表示药物的名词搭配。influential 有影响的, 有势力的, monstrous 异常大的, vigorous 精力旺盛的,