题目

同温同压下，同体积的气体单质X2和气体单质Y2化合生成气体Z，且反应前后气体体 积不变，则Z的化学式为 A．X2Y2         B．X2Y         C．XY2           D．XY 答案：D一Why were n’t you at the meeting?一I for a long distance call from my sister in America·A．waited B．was waitingC．have waited D．had waited