几道英语问题。。不知道怎么解释？。。
-The pen writes well though it doesn't cost much.
-Let me have a try. So____b_____.
a.it is
b.it does
c.does it
d.is it
-___a___ is the population of the town?
-Over 20000. And a third of the population _____ workers of the car factory.
a.What,are
b.How many,are
c.What,is
d.How many,is
-Go and ask the waiter how much ____d___
-Don't worry. It has already been________
a.does the meal cost,paid for
b.the meal costs,paid
c.the meal spends,paid
d.the meal costs,paid for
虽然我知道答案，但是不懂为什么？知道的快告诉我啊。谢咯。。
参考答案:1、选B原因：前一句说钢笔好用但不贵，第二句说试一下，确实好用。so it does和so it is都是不完全倒装表示进一步认同，“确实是”的意思。so does it和so is it是完全倒装表示“……也是”，这里可以首先排除a，b，因为the pen writes well中的"write"是行为动词而不是be动词，因此相应的助动词应用do而句中又是单数writes因此要用does而非is，然后再根据 so it does和so does it意思上的区别最后选出正确答案。
2、选a：what is the population of……是提问人口多少的固定说法，记住即可。第二个空则可根据workers来确定：a third of the population要与workers构成逻辑上的对应因此可推出前者也表示复数，所以选are
3、选d：how much the meal costs作ask后的宾语从句，可以确定第一空为陈述句语序，排除a；cost 的主语一般为所消费的对象（即meal），spend 的主语一般为消费的主体，可排除c；It has been paid for，是个被动语态句子，for是必须的，因为它与该句前面的It（代替meal）构成介宾关系。