I was out of town at the time，so I don’t know exactly how it ________.
| A．was happening | B．happened |
| C．happens | D．has happened |
He was about to leave the office for home _______someone knocked the door and asked for some help.
| A．until | B．while | C．as | D．when |
There are nearly one hundred foreign students in our university, most of _______ are from America and Australia.
| A．which | B．them | C．whom | D．those |
The climbers succeeded in climbing to the top of the mountains, and they owed their success to the preparations they ______.
| A．make | B．had made | C．made | D．have made |
The students expected there ____more reviewing classes before the final exams.
| A．to be | B．are | C．being | D．is |
Such things ___ you described in the report seldom happen now.
| A．that | B．who | C．as | D．which |