题目内容：

1.I am busy now,so I can’t help ______ the machine.A.repair B.to have repaired C.repairing 实在不知道怎么选a
2..The boy seated himself in the corner with his back ______ to his father.
A.turning B.to turn C.to be turned D.turned为什么不能选turning而要选turned,这里有要表过去的意义吗
3..______ the room,a letter was laid on the ground.
A.Entering B.Having entered
C.He entered D.Mrs Green entering觉得前三个选项都不对,因为主句的主语是letter,但是对于选d的用法也不明白
4.______ to nuclear radiation,even for a short time,may produce variants of genes in human bodies.
A.Being exposed B.Having exposed
C.Exposed D.After being exposed 这个题更让我迷糊,主句的主语没找到,还是说主句的主语在分词短语里,是什么在核辐射下暴露了在短时间内就产生了人体基因的变异?
5.----Will you please tell us how you can seize the robbers?
----I will spend a whole week ______ in your room to wait for their coming.
A.locking B.to lock C.locked D.being locked
不明白为什么选c,即使是被锁,为什么不能是d