＿＿＿used to be thought that the earth was square.
| A．He | B．What | C．It | D．That |
______ 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 |
Doing your homework is a sure way to improve your test scores, and this is especially true______ it comes to the classroom tests.
| A．before | B．as | C．since | D．when |
Substances will expand or contract ______ heated or cooled, but this is not the case with water.
| A．when they will be | B．if they will be |
| C．as they being | D．when |
You can arrive in Beijing earlier for the meeting __ you don’t mind taking the night train.
| A．provided | B．unless | C．though | D．until |