单项选择（每题1分）
We must think twice _____ we make the final decision.
| A．when | B．while | C．until | D．before |
It is pretty well understood __________controls the flow of carbon dioxide in and out the atmosphere today.
| A．that | B．when | C．what | D．how |
____is certain is ___prevention is more important than treatment .
A．It;that  |
B．What;that | C．As;what | D．What;what |
_______has recently been done to provide more buses for the people ,a shortage of public vehicles remains a serious problem .
| A．That | B．What | C．In spite of what | D．Though what |
The experience of the Chinese nation attests to a truth____ a nation loses in times of disaster will be made up for by her progress
| A．that what | B．what | C．that | D．what that |
In his speech, Premier Wen Jiabao points out that creativity is ______it takes to keep a nation highly competitive .
| A．how | B．what | C．which | D．that |