题目

根据元素在周期表中的位置判断，下列元素中原子半径最小的是        A．氧           B．氟          C．碳          D． 氮  答案：B解析:考查元素周期律的应用。4种元素均属于第二周期，同周期元素自左向右原子半径逐渐减小。4种元素F的原子序数最大，其半径最小。所以答案是B。 Children get tired of learning often because they ________ to learn more and better than they can. [　　] A．expectB．are expected C．have expectedD．are expecting