38.―Look.how happily the boys are swimming in the pool!―How long they ?―For about an hour.A.have.been swimming B.have.swumC.do.swim D.have.be swimming 查看更多

 

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—Look,how happily the boys are swimming in the pool!

—How long ________ they ________?

—For about an hour.

A.have,been swimming                             B.have,swum

C.do,swim                                                 D.have,be swimming

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