Try the back door if nobody answers the front door. A．to knock at B．knocking at C．an——青夏教育精英家教网—

摘要：Try the back door if nobody answers the front door. A．to knock at B．knocking at C．and knock at D．and knocking at

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Try________at the back door if nobody answers you at the front door.

A．to knock B．to be knocking

C．knocking　 D．having knocked

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