题目

By serving others, a person focuses on someone other than himself or herself, ____ can be very eye-opening and rewarding. A.who                        B. which                            C. what                             D. that 答案：B下列各元素中，一定属于主族元素的是   A.X元素能形成+7价的含氧酸及其盐   B.Y元素原子最外层电子数为2   C.Z元素的阴离子与同一周期稀有气体元素的原子电子层结构相同   D.R元素的最高价氧化物是酸性氧化物