题目

阅读下面材料,根据其内容和所给段落开头语续写两段，使之构成一篇完整的短文。续 写的词数应为150左右。Christmas was hanging around. Six-year-old David was thinking what presents he could get from Santa. He wanted slippers; he wanted a silk tie; he wanted some toys. And school starting soon, he also hoped to get something about schooling. In order to get Santa to know what he needed, he thought he would write a letter to Santa, which he believed would bring him those presents. He found a sheet and made a list of the items on it, and then bent down to pos密闭容器中有反应：Mg+2HCl=MgCl2+H2↑ 2s后HCl的浓度由2mol/L减小到1mol/L,求该反应的速率 （ ） A．1mol/ (L.s) B．0.5mol/(L.s) C．1.5 mol/(L.s) D．无法计算