_______cloning human embryos is not legal in many countries, some scientists are already pushing ahead with research in order to produce a cloned human baby.
| A．While | B．When | C．Since | D．if only |
The oil must have been used up, _____ the light went out.
| A．for | B．because | C．since | D．as |
It is only when you lose something _____ how much you value it.
| A．then you will realize | B．do you realize |
| C．that you will realize | D．which you will realize |
A new policy formulated, _____.
| A．thus did the economic reform begin |
| B．thus began the economic reform |
| C．thus had the new economic reform began |
| D．began the economic reform thus |
_____ this something, still unseen, they gave the name radium.
| A．With | B．By | C．At | D．To |
Look out! The ceiling _____ l down.
| A．is to fall | B．will fall | C．is going to fall | D．is falling |