At the beginning of class, the noise of desks ________ could be heard outside the classroom. A. opened and closed B. to be opened and closed C. being opened and closed D. to open and close 答案:C在一个不透明的口袋里装有只有颜色不同的黑、白两种颜色的球共20只,某学习小组做摸球试验,将球搅匀后从中随机摸出一个球记下颜色,再把它放回袋中,不断重复.下表是活动进行中的一组统计数据:
摸球的次数n
100
150
200
500
800
1000
摸到白球的次数m
58
96
116
295
484
601
摸到白球的频率mn
0.58
0.61
0.58
0.59
0.605
0.601根据表中的数据可估算出口袋中白球约有1212只.