不定项选择

抽样误差是指由于抽样的随机性引起的样本结果与总体真实值之间的误差

A．正确
B．错误

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单项选择题

Suppose there are two discrete channels, their channel matrices are P1=[1/2 1/2 ; 1/3 2/3] and P2=[1/3 1/3 1/3; 1/2 0 1/2], then the cascade channel matrix is ( ).
A、P=[1/2 1/2; 1/3 2/3]
B、P=[1/3 1/3 1/3; 1/2 0 1/2]
C、P=[5/12 1/6 5/12; 4/9 1/9 4/9]
D、P=[1/2 1/2; 5/12 7/12; 1/3 2/3]

当对方用左直拳进攻我面部时，可采用后手的拍格挡防守反击左直拳。请判断（）

答案：错误