---It’s a long time _____ I saw you last.
---Yes, and what a pity it is now that it will be a long time _______ we see each other again.
| A．before; since | B．since; when | C．since; before | D．when; before |
_____ human beings live in great numbers, the waters are very likely to become contaminated.
| A．That | B．Where | C．Even if | D．Though |
_____ I admit that the problems are difficult , I don’t agree that they cannot be solved.
| A．While | B．Suppose | C．Because | D．as though |
It was found that although the girl could perceive things with her fingers, this ability ceased _____ her hands were wet.
| A．the moment when | B．the moment | C．in that moment | D．before |
______ you understand this rule, you’ll have no further difficulty.
| A．while | B．Once | C．Though | D．Unless |
It will not be long _____man can harness the solar energy and make it serve mankind.
| A．since | B．before | C．when | D．after |