问题
选择题
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
答案
答案：A
题目分析：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.