几道英语练习题（被动语态）不懂1.These children____A___ dance.A.were seen to

几道英语练习题（被动语态）不懂
1.These children____A___ dance.
A.were seen to B.were seen for C.were seen D.saw to
(see是系动词,无被动的吧? 还有为什么加to)
2.Good care ___B___ such things.
A.should be taken B.should be taken of
3.Now these magazines ___D__ in the library for a long time.
A.have kept B.are keeping C.have been keeping D.have been kept
(keep是系动词无被动语态的把?为什么选D）
4.-I'd like to buy that coat
- I'm sorry.__C____
A.it sold B.it's selling C.It's been sold D.it had been sold
(sold强调状态,无被动语态的吧?为什么选C,这道不懂）
5.The key ___B___ on the table when I leave.
A.was left B.will be left C.is left D.has been left
(leave是不及物动词吧?无被动语态的吧?为什么?）
6.What you said like a good idea.
A.heard B.listened C.sound D. sounded
马上要升高中,这些联系题是高中和初中衔接的,所以很多不懂,希望大家能帮我解析一下!