题目

口袋里放有大小相同的2个红球和1个白球，有放回的每次模取一个球，定义数列 如果为数列的前n项之和，那么的概率为                                                                         （ ）     A．          B．          C．         D． 答案：B________ anything about what had happened, he went to work as usual.A.Don't knowB.Not to knowC.Not knowingD.Not to be knowing