【观察】观察下列几组句子中it的用法，然后加以总结。
1. a. It was for this very reason that Mr. Li came to work late this morning.
b. It was a professor from Peking University who / that gave us a speech yesterday.
2. a. It must be the naughty boys who are making so much noise.
b. It might have been at midnight that the traffic accident took place.
3. a. Is it in this factory that your father worked 20 years ago?
b. Was it on the bus that you had your cellphone stolen yesterday?
4. a. What is it that your boyfriend has bought you as a birthday present?
