1. It is not easy ____ the answer to the difficult math problem.

A. to figure out

B. figuring out

C. figure out

D. being figured out

【答案】A. 本题考查的是it 作形式主语，真正的主语是to do。这种句型很常见，例如：It’s difficult to find a job these days. 如今找一份工作很难。

2. He has already made up his mind. It’s no use_____ him.

A. try to persuade

B. trying to persuade

C. to try to persuade

D. to try persuading

【答案】B It is no use doing sth.“作某事没有用”是固定搭配。Ing做真正主语。

3. ____ is one of good properties of this metal.

A. Light in weight

B. Having been light in weight

C. Be light in weight

D. Being light in weight

【答案】D 主语应该是名词性质，因此排除A项，因为light 是形容词;也排除C 项，be light in weight 不能作为名词词性;而此句陈述的是客观真理，应该用一般现在时而不是完成时， 所以选D项。D项意为“质量轻 ”，in 表示“在某方面 ”，weight “质量，重量 ”;句意为“重量轻是这种金属的优良特征之一。”

4. It's necessary for them ____ another chance.

A. to give

B. to be given

C. being given

D. giving

【答案】B 再给他们一次机会是很重要的。不定式作主语。

5. It's necessary to be prepared for a job interview. ________ the answers ready will be of great help.

A. To have had

B. Having had

C. Have

D. Having

【答案】D 面试之前有必要做好准备，提前准备好答案会很有帮助。Ing形式做主语。