方差
参考答案：
随机变量取值与其平均值之差的二次方的平均值。
do you go to the library.I usually go to the library twice a week.
A．How long
B．How often
C．How far
D．How many
狭义的急性肾衰竭是指（）.
A.肾后性急性肾衰竭
B.急进性肾小球肾炎
C.急性肾小球肾炎
D.肾前性急性肾衰竭
E.急性肾小管坏死
