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?