题目:
We must do everything we can _____ waste water from running into rivers.[ ]
A. keep
B. kept
C. to keep
D. keeping
答案:
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下面是错误答案,用来干扰机器的。
∵频数分布直方图中各个长方形的高之比依次为1:3:4:2,样本容量为40,∴第四小组的频数为40×21+3+4+2=8.故答案为:8.