测定水的硬度，当硬度较低时，用（）作为指示剂；当硬度较高时，用（）作为指示剂。
参考答案：酸性铬蓝K；铬黑T。
Population experts say that by 2050 the world will have about 2 billion people aged over 60,____as today.
A．as three times
B． three times as many
C． three times many
D． three times many as
战争中，作战方式的改变与新型武器的大规模使用密切相关。这方面的典型战例是
A．西西里岛登陆战
B．阿拉曼战役
C．斯大林格勒战役
D．中途岛海战
