问题补充：
Isnecessary to change trains at Beijing?
A. this
B. that
C. it
D. he
答案：
A. this
B. that
(答案→)C. it
D. he
解析：不定式、动名词作主语时常用形式主语it来代替，而把真正的主语──不定式和动名词后置。常住it作形式主语的动名词结构有：It’s no use (no good, no point) doing sth. It’s a waste of time doing sth.等。故本题选C。又如：It was no good quarrelling again.
