我这里有几道英语难题，请各位前辈帮忙详解，谢谢！
1.How does it happen that the two-party system is so strongly( )(root)in American politics?
2.Economics.several courses of which i have taken thus far,( )(prove)to be difficult but useful for almost all students.
3.After( )(interview)for the job,you sill be required to take a language test.
4.The petrol tank is empty.i( )(fill)it up before we left home.
5.Lack of concentration is a sign of your( )(able)to overcome the distracters.
这几个题目是要求在空白处填上括号里所给词语的正确形式，看到这种题目，我有点晕，不知道应当怎样分析和下笔，请老师帮详细解答并翻译成中文，好吗？谢谢！

1.How does it happen that the two-party system is so strongly( )(root)in American politics? 为什么两党政策在美国那么根深蒂固？
2.Economics.several courses of which i have taken thus far,( )(prove)to be difficult but useful for almost all students.在我目前所选的几门课程当中，经济学被认为是对几乎所有学生都很困难而又实用的一门课。
3.After( )(interview)for the job,you sill be required to take a language test.职务面试过后你将被要求参加一个语言测试。
4.The petrol tank is empty.i( )(fill)it up before we left home.油箱空了，我本该在出门之前加满油的！
5.Lack of concentration is a sign of your( )(able)to overcome the distracters.注意力不够集中是你在克服困境时的一处硬伤。
1。rooted
2. has been proved
3. interviewing
4. should havefilled
5. disability

第三题，我又仔细想了一下，确实有些棘手！如果用名词形式，那么至少前面应该加上一个冠词才好啊。各位看看能不能填having interviewed?