The flu is believed by viruses that like to reproduce in the cells inside the human nose and throat.
A. to be caused
B. to have caused
C. being caused
D. causing
网友回答
(答案→)A. to be caused
B. to have caused
C. being caused
D. causing
解析：句意：流感被认为是由在人类鼻腔和喉腔细胞内繁殖的病毒引起的。名词+be+believed/said/reported+不定式”相当于“It is believed/said/reported that…”，可以看作是一个句型。flu与cause之间是一种动宾关系，所以应该用动词不定式的被动式，选A.