If in, the viruses can result in illness or even deaths.
A. breathed
B. to breathe
C. breathes
D. is breathed
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(答案→)A. breathed
B. to breathe
C. breathes
D. is breathed
解析：考察非谓语动词。Breath的逻辑主语是viruses，被动关系，所以选择breathed=the viruses are breathed in,
句意为如果被吸入体内，这些病毒能致病或者致死。