题目内容：

It has been a lesson to mankind that immediate action must be taken the moment some new virus____.
A.is happened B.will happen C.happens D.would happen

优质解答

c.the moment 相当于when..前面用HAS BEEN现完说明还是现在时.
句子是说当新病毒传播时必须马上采取措施对人们来说已经是一个教训~~.
(*^__^*) 嘻嘻……