The changes that howe taken place place in air travel during the last sixty years would have seemed completely impossible to even the most brilliant scientists at the turn of the 19th century.

　　It is four years since John left school.

　　（4）在It is the+序数词/形容词最高级+that的定语从句中，谓语动词常用现在完成时。如：It isn''t the first time that I have found myself in an embarrassing situation.

　　（5）在no sooner…than…，hardly/scarcely…when/before…句型中，主句常用过去完成时。

　　（6）其他与完成时连用的时间状语：all this while，all this year，for some time，so far，already，before，just，long，yet等。

　　四、完成进行时指动作在完成时的基础上还要继续下去

　　The company has been promising a rise in salary for ages，but nothing has happened.

　　The school board listened quietly as John read the demand that his followers had been demonstrating for.

　　时态、语态答题思路：

　　（1）先根据选项的区别点确定考题要点为时态，然后回到题句中寻找给出的或暗示的时间状语，缩小选择范围，进而选出正确答案；

　　（2）根据谓语动词与句子主语或非谓语动词与其逻辑主语的关系，确定句子是主动语态还是被动语态。

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