题目

按体积分数计算，氧气在空气中约占（ ）                        A．78%              B．21%              C．0.94%            D．0.03% 答案：【考点】空气的成分及各成分的体积分数．                          【专题】分析比较法．                                         【分析】根据空气的组成判断，空气的成分按体积计算，大约是氮气78%、氧气21%、稀有气体0.94%、二氧化碳0.03%、其它气体和杂质0.03%． —Why are you standing there, Kangkang?—I can’t see the blackboard clearly. Two tall boys are sitting _________ me.A. behind          B. next to                 C. in front of