单项选择(20 小题，每小题1分，共20分)
从A、B、C、D四个选项中选出一个最佳答案，并将其标号填入题前括号内。
( )21．一Be careful， you will make mistakes in your exams．
--I know that，Mum．One can never be too careful．
A．and B．or C．nor D．but
( )22．1 wanted to explain，but he didn’t give me any ．
A. excuse B．chance C．way D．choice
