问题
选择题
The flu is believed ________ by viruses that like to reproduce in the cell inside the human nose and throat.
A．causing
B．being caused
C．to be caused
D．to have caused
答案
答案：C
在“sb/sth is/was＋过去分词＋不定式”句型中，如果不定式所表示的动作在谓语动词后面发生，动词不定式要用一般式。