题目

Occasions are quite rare ____ I have the time to spend a day with my kids.A. who B. which C. why D. when 答案：【答案】D【解析】答案：D。考查定语从句。句意：我难得有时间和孩子们一起过一天。先行词为occasions，表时间。在10件产品中有2件次品，任意抽取3件，则抽到次品个数的数学期望的值是______．