题目

The number of the children in the school ________ about three hundred. ________ of them________ from the countryside.A. is; 20 percents; are  B. is; 20 percent; are C. are; 20 percent; is   D. are; 20 percents; are 答案：B下列叙述正确的是 （　　） A、0.1mol/L CH3COONa溶液中：c（Na+）＞c（CH3COO-）＞c（H+）＞c（OH-）B、Na2CO3溶液加水稀释后，恢复至原温度，pH和Kw均减小C、pH=5的CH3COOH溶液和PH=5的NH4Cl溶液中，c（H+）不相等D、在Na2S溶液中加入AgCl固体，溶液中c（S2-）下降