计划期天数,指计划期的日历天数或周期数,一般一年按()周计。
A.40
B.50
C.52
D.53
参考答案:C
I often hear the ____ of children playing happily downstairs at this time of day. [ ]
A. voice
B. noise
C. sound
D. throat
设离散型二维随机变量(X,Y)的取值为(xi,yi)(i,j=1,2),且
试求: (Ⅰ)二维随机变量(X,Y)的联合概率分布; (Ⅱ)X与Y的相关系数ρXY; (Ⅲ)条件概率PY=yj|X=x1,j=1,2.