题目

下列各组分子中属于含极性键的非极性分子的是( ) A．CO2、H2S                             B．C2H4、CH4 C．Cl2、C2H2                            D．NH3、HCl 答案：解析 A选项中，H2S为含有极性键的极性分子；C选项中，Cl2为含非极性键的非极性分子；D中NH3和HCl都为含极性键的极性分子；B选项中两种分子都为含极性键的非极性分子。 答案 B After Jack had sent some e-mails, he ________ working on his project． [　　] A．had started B．has started C．started D．starts