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have been introduced have introduced are introducing are introduced
will visit am going to visit will be visiting would visit
produced being produced to be produced having been produced
did had done was doing have done
Given To give Giving Having given
came；would meet had come；would have met come；will meet had come；would met
a光的波长小于b光的波长 a光的频率大于b光的频率 在该玻璃砖中，a光的传播速度比b光大 在真空中，a光的传播速度比b光大
is are have been has been
knowing know to know being known
the : the an :不填 an : the the : a
What；what That；that What；that That；what
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