问题描述：
一道关于try to do和try doing用法的英语题!
No body answer the front door.So he tried ____ at the back door.
A.to knock B.knock
C.knocked D.knocking
应该选择哪个?
No body answer the front door.So he tried ____ at the back door.
A.to knock B.knock
C.knocked D.knocking
应该选择哪个?
问题解答：
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