一般装有遮光栅格的灯具，其效率不宜低于（）。
A.70％
B.65％
C.55％
D.50％
参考答案：C
______ I admit that the problems are difficult, I don't agree that they can't be solved.[ ]
A. In case
B. While
C. As
D. Because
两带轮传动，其传动比i＝0.5，则它传动是（）传动
A、减速
B、增速
C、等速
