The flu is believed ______ by viruses that like to reproduce in the cells inside the human nose and throat.
A．to be causedB．to have causedC．being causedD．causing
A

試題分析：句意：流感被認(rèn)為是由在人類鼻腔和喉腔細(xì)胞內(nèi)繁殖的病毒引起的。名詞+be+believed/said/reported+不定式”相當(dāng)于“It is believed/said/reported that…”，可以看作是一個句型。flu與cause之間是一種動賓關(guān)系，所以應(yīng)該用動詞不定式的被動式，選A.
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