单选题 A light sleeper is usually very ______ to any sound even as inaudible as the humming of a mosquito.

【正确答案】 C
【答案解析】alert机警的，机敏的，be alert to为固定短语，对…很警觉。acute与keen都有“敏锐”之意(如目光、思维等)。immune免疫的。