They can strike anywhere, anytime. Wherever we go and whatever we do, we are approached by invaders from an unseen world. A menagerie of microscopic pests constantly assaults every part of our bodies, __20___ for a way inside. Many are harmless or easy to fight off; __21___ are merciless killers. And for all the vaunted power of modern medicine, deadly infections are __22___ threat. Just 30 years ago, medical researchers boasted that victory against infectious diseases was simply __23___. That thinking was demolished by the advent of a number of diseases, particularly AIDS. The question is no longer: When will infectious diseases be wiped out? __24___, it is: Where will the next deadly new plague appear?
【題組】21. (A) other (B) another (C) others (D) the others
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