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【单选题】
一般石英砂过滤的冲洗强度为12~15L（s²m2），水冲的时间为（）min。
A.
2~1
B.
3~2
C.
5~3
D.
7~5
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参考答案：
参考解析：
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举一反三
【单选题】When stars like our Sun die, they bloat to become red giants and then eject gigantic clouds of gas and dust into space. Increasingly, however, scientists found themselves at a profound loss to explain...
A.
explain-the possible way of death of our Sun.
B.
show the two types of red giants.
C.
discuss on the way of red giants to become white dwarfs.
D.
explore the origin of life.