题目

— ________? —She is twenty． A．What's your grandmother's age                 B．How old is your brother C．How old is your sister                                D．When is your sister's birthday 答案：C已知一组数据x1，x2，x3，x4，x5的平均数是2，方差是，那么另一组数据3x1－2，3x2－2，3x3－2，3x4－2，3x5－2的平均数和方差分别是 [　　] A．2，B．2，1 C．4，D．4，3