This visit will greatly ____ our understanding of your great country.
| A．add up to | B．add up | C．add to | D．add in |
Although he knew little about the large amount of work done in the field , he succeeded ____other more well-informed experimenters failed.
| A．as | B．unless | C．what | D．where |
John waited at the bus-stop for nearly half an hour _____ the bus finally arrived .
| A．when | B．as | C．before | D．while |
Not that I’m unwilling to lend you a hand , ____I’m too busy for the moment .
| A．because | B．but that | C．but | D．however |
Jack had traveled six miles across the Channel _____his engine failed and was forced to land on the sea .
| A．when | B．until | C．after | D．since |
—When did he leave the classroom?
—He left _____you turned back to write on the blackboard.
| A．the minute | B．the time | C．until | D．before |