After a knock at the door, the child heard his mother’s voice ______ him.
| A．calling | B．called | C．being called | D．to call |
I had hardly got to the office ______ my wife phoned me to go back home at once.
| A．when | B．than |
| C．until | D．after |
I have heard a lot of good things about you ________ I came back from abroad.
| A．since | B．until |
| C．before | D．when |
There's no way of knowing why one man makes an important discovery ________ another man, also intelligent, fails.
| A．since | B．if |
| C．as | D．while |
There are some health problems that, when ________ in time, can become bigger.
| A．not treated | B．not being treated |
| C．not to be treated | D．not having been treated |
In the global economy, a new drug for cancer, ________ it is discovered, will create many economic possibilities around the world.
| A．whatever | B．whoever |
| C．wherever | D．whichever |