【

分析

】答案选C，为 if it is carefully done 之省略。

3.

The research is so designed that once _________ nothing can be done to change it.

A. begins                 B. having begun    C. beginning               D. begun

【

分析

】答案选D，为 once it is begun 之省略(from http://yingyu.chazidian.com)。

五、虚拟语气省略陷阱

在虚拟条件句中，有时可将引导条件从句的连词if省略，而将从句中的were, should, had 等提到句首。如：

Were I Tom, I would refuse. 如果我是汤姆，我会拒绝。

I will go, should it be necessary. 假若有必要，我会去的。

Had it not been for the bad weather we would have arrived in time. 若不是天气坏，我们就准时到达了。

值得注意的是，若条件从句为否定句，否定词not应置于主语之后，而不能与were, should, had 等缩略成Weren’t, Shouldn’t, Hadn’t而置于句首。另外，有时省略if后提前的had不是助动词。如：

Had I time, I would come. 假若我有时间，我会来的。（=If I had time…）

请看一道试题：

It _________ very silly to argue any more against each other about it _________ true.

A. will be; could that be                              B. must be; could that be

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