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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