1.“where did the accident happen?”It was at the crossing of Siping Road and Guoquan Road____the traffic was heavy then.”
a.that b.where
2.To what extent will____buses ____the decrease of the pollution?
a.environment ;be made friend
3.The doctor have tried __to save his life.
a.humanly everything possible
b.everything humanly possible
4.-Has he decided to come tomorrow?
-He said he ___but it depended on the weather.
a.could b.might c.would d.should
5.Our school building is quite different from ___it was before.
a.that b.which c.what d.this
6.____,our next step is to determine how to carry it out.
a.the plan having been made b.having made the plan
麻烦解释一下原因

1B
省略了后半部分的强调句，同时插入了一个定语从句修饰。选的是一个引导定语从句的关系副词。

2B
friendly是形容词；make（使）的逻辑宾语是decrease而不是bus，所以不用被动。

3B
修饰every、some、any、no＋one、body、thing、where构成的不定代词，形容词要后置，humanly修饰的是possible，所以应当紧跟在其前后。

4C
注意不要误选might，认为是表示可能，这里不可能是表示猜测的用法，因为他不需要对自己的行为猜测。

5C
what引导的是一个介词from的宾语从句，what在从句中充当表语（位于be之后），所以只能用what。

6A
句子是一个独立主格结构，因为非谓语动词make的逻辑主语是plan，而句子的主语是step，两者不一致，必须用“逻辑主语＋非谓语动词”的独立主格结构。