---He looks unhappy.

---Yes, but he can’t say what it is __ is bothering him.

a. as b. what c. that d. which

2. They shouted for help ___ .

a. as loudly as they can b. as loudly as possible c. as loudly as they could

3. Those who were left alone in the cave know better than the others __ sounds bats in the dark made.

a. that frightening b. which frightening c. what frightening

请回答为什么?

1. c.
这里不是定语从句，否则 c,d 都是正确答案。而是it 的强调句型。it is ...that 如：tom helped me with my englsih yesterday.
it was tom that helped me with my englsih yesterday.(强调tom ,而不是别人)
it was yesterday that tome helped me with my englsih .(强调yesterday ,而不是其它时间)
这里应为用了what, 所以把what 放在it 前 .

2 c.
a中用了can ， 因为是过去时，所以不对 a ,c 类似， 从技巧角度讲，答案 有可能数其中之一
从语感角度看， c要比b好，他们尽自己所能大声呼叫求助。能－－could
类似的： run as fast as you can
he recited as much ofthe poem as he could remember

3。 c
类似1。 这里不是定语从句修饰或指代一个名词，否则 a b都是正确答案。
what引导的定语从句从know 的宾语
那些独自留在山洞里的人，比其他人更清楚蝙蝠在黑暗中发出的是怎样可怕的声音。
这句话中， 真正的定语从句修饰或指代一个名词是sounds （that/which）bats in the dark made
类似的：i do not know what you are talking.
i do care what you think of me .