try (knocking)at the door once more为什么不填knock?他不是祈使句吗?祈使句不是一定要用一般时?

27 If nobody answers the door bell ,try ___ the back door.A to knock at b.to knock c.knocking at d.knocking 需要详细分析 请教英语达人这句英语句子可否这样书写?原句：Try knocking at the back door if nobody hear you at front door.可否这样书写：If nobody hear you at front door try knocking at the back door. There seems to be no one at home -Why not()at the back door A.to try knocking B.to try to knockC.trying to knock D.try knocking try (knocking)at the door once more为什么不填knock?他不是祈使句吗?祈使句不是一定要用一般时? someone is knocking at the door.It might be the posyman.为什么用is knocking 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.knockC.knocked D.knocking应该选择哪个? There is someone knocking at the door.为什么用“knocking”,要加“ing”? i seem to hear someone knocking at the door,knocking为什么要加ing 为什么是is knocking：Who is knocking(knock) at the door?it's me. Don't come in before knocking at the door.before 后面的 knocking用的对吗? 单句改错 who is the men knocking at the door Someone is knocking at the door.____ is it? I tried knocking at the front door but nobody ___it Listen!_______(有人）is knocking at the door. Someone is knocking at the door,but who can____ be? Please do not e_____without knocking at the door. Please do not ------without knocking at the door怎么填 If no one answer the door,why not try knocking the back door?