在破碎筛分系统中，筛子选择至关重要，最好选用长、宽比为（）。
A、≧1
B、≧2
C、≧3
参考答案：C
用所给单词的适当形式填空。
1. The river ______ (look) so beautiful in summer.
2. Mum, remember ______ (buy) a new jacket for me.
3. My hair ______ (be) short.
4. Can you hear somebody ______ (sing) in the garden?
5. Does your mother like ______ (run) ?
—May I go out to play soccer ball?
—I’m afraid not. You can’t play __ you have finished your homework.
A．after
B．when
C．of
D．unless
