请说明理由1,these trees must have been planted three years ago.__

请说明理由
1,these trees must have been planted three years ago.___?
a.weren't they?
b.must'nt they
c.didn't they
d.haven't they
2.an iron and steel works ,with some satelite factories,__to be built here
a.are going
b.is going
c.shall
d.will
3.the boy and girl each___toys
a.has her own
b.has their own
c.have their own
d.have his own

1.c.该题是推测性题的反问,应根据后面的three years ago,所以是过去式
2.b 这题把主语与谓语分离了,iron and steel 是一个概念,钢铁,works是工厂的意思,所以应该用单数.
3.c each 再这里表示的不是整体的概念而是个体的概念,故用复数形式,选c 楼上那位明显自我矛盾
再问： 第三题既然wach是个体概念为什么还是复数呢
再答： 哦，each虽然是个体概念，但是代表几个不同的个体，从意义上来说就是复数了，而整体概念的话就是一个集体，当然是一个单数，我们要用英语思维来考虑这个问题，有时候有些不习惯。
再问： 那第一题不是mustn't they而是weren't they呢.. 这个表被动我能想得通,但mustn't也没错啊...
再答： 呵呵，对，我第一题错了，应该是答案A，对已发生的过去情况的推测，若陈述句谓语部分有“must have done”，而且有表示过去的时间状语，问句部分用didn't；若没有表示过去的时间状语，问句部分用haven't或hasn't。如本题有three days ago，所以用were't they?而没有用must 这个形式。另附其他一些形式的答题技巧！希望能帮助你~ 一、对现在情况的推测，问句部分用主动词（do,be)一般现在时的适当形式。若是现在进行时，问句部分用现在进行时的适当形式表示。若是there be结构，问句用isn't/aren't there。如： 1)He must be there,isn't he? 2)He must have a big family,doesn't he? 3)He must be waiting outside,isn't he? 4)There must be some students in the room,aren't there? 二、对已发生的过去情况的推测，若陈述句谓语部分有“must have done”，而且有表示过去的时间状语，问句部分用didn't；若没有表示过去的时间状语，问句部分用haven't或hasn't。如： 1)They must have gone there last night,didn't they? 2)They must have arrived by now,haven't they?（根据by now来判断） 3)They must have been to the Great Wall,haven't they? 三、若是被动，应按被动结构来处理。如： 1)The room must have been cleaned yesterday,wasn't it? 2)The room must have been cleaned,hasn't it? 四、若句中有表示过去完成时的时间状语，问句部分应用hadn't.如: They must have learnt 5000 English words by the end of last term,hadn't they? (本题中must表推测，如果将它去掉，还原为真实句就是They had learnt 5000 English words by the end of last term.因此，反意问句是hadn't）