30.They ______ be at home. You see, the door is locked from the outside.
| A．can’t | B．may | C． must | D． mustn’t |
On the other side of the street stand more than 180 houses, _______are in poor condition now．
| A．of which half | B．which half of |
| C．half of them | D．of them half |
I first met Mr Smith in America. He ________ at Stanford University then.
| A．studied | B．had studied |
| C．is studying | D．was studying |
The book isn't intended for general readers probably because the author _______ too many scientific terms(术语) in it.
| A．wrote | B．employed | C．analyzed | D．introduced |
---- What does P．O．_________?
----Poster order, I guess．
| A．call for | B．fall for |
| C．prepare for | D．stand for |
It was not until dark __________ he found __________he thought was the correct way to solve the problem．
| A．that; what | B．that; that |
| C．when; what | D．when; that |