常见的高一定语从句的问题...（答得好有加分）

常见的高一定语从句的问题...（答得好有加分）
以下选择题请选出答案并解释其他项不选的理由（最好还有错项怎样改才是对的）
还有一些定语从句的语法问题..希望有高手可以慢慢地解释..（最好加上例句）
PS：只想好好听听各位高手的理解...
1.Delia's going to join us,______ was agreed on the day before yesterday.A.it B.that C.what D.as
还有as在定语从句中的用法是怎样的?它修饰的是主语宾语还是一个句子呢?修饰一个句子的时候是不是和which 相同呢?as在定语从句中一般译为什么呢?
2.You can use a large plastic bottle,_______ cut off,as a pot to grow flowers in.
A.the top is B.with its top C.whose top D.the top of which
同位语从句是什么?怎样判断一个从句是同位语还是定语从句?with its
top cut off 是不是等于 its top which is cut off?
3.When you read the book,you'd better make a mark ______ you have any questions.
A.at which B.at where C,the place where D.where
如何判断一个句子是定语从句还是状语从句?两个从句的概念是什么?
C项怎么改才是正确的呢?
4.Such a man _______ you described doesn't seem to have existed.
A.which B.that.C.as D.what
such as 和 such that 的区别是什么?是不是有so .such the same
在定语从句中就一定用as呢?
5.As a teacher,I seldom give my student so difficult a problem ______ they can not work out.
A.that B.if C.in order that D.as
如果这个句子用so that 的话应该怎么改呢?