题目

水是地球上最普遍、最常见的物质之一，人类的日常生活离不开水。下列有关水的说法正确的是    A．水蒸发后，化学性质发生了变化    B．水电解生成氢气和氧气，说明水中含有氢分子和氧分子    C．冰块与水混合得到混合物    D．明矾可用于净水  答案：D解析:略 33、Only after I had known him for some time      to appreciate his real worth.        A. do I begin                                         B. did I begin        C. I began                                             D. had I begun