光调制器的作用是实现（）。
A.光电转换
B.电光转换
C.光放大
D.电流放大
参考答案：B
34. Catherine used to _____ much at lunchtime, but recently, in order to have a good figure, she has got used to ____ less at noon..
A．eating; eat
B．eat; eating
C．eat; eat
D．eating; eating
用直接法配制标准溶液的物质，必须具备什么条件？
