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.