A:镜花水月
B:海市蜃楼
C:坐井观天
D:立竿见影
某批乒乓球的质量检验结果如下:
抽取的乒乓球数n
200
500
1000
1500
2000
优等品频数m
188
471
946
1426
1898
优等品频率mn
0.940
0.942
0.946
0.951
0.949
Today is C____ Day. We are very happy.