There is ____ Mr. Smith at the door who

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问题选择题

There is ____ Mr. Smith at the door who wants to speak to you.

A．a some

B．certain

C．some a

D．a certain

答案

答案：D

填空题

想一想，下面的字母减去一笔就会变成哪个字母呢？把它们写下来。

1. E → ______________

2. B → ______________

3. M → ______________

4. G → ______________

5. N → ______________

6. Q → ______________

计算题

（2x²y﹣4xy²）﹣（﹣3xy²+x²y），其中x=﹣1，y=2．