题目

— I was about to leave home _____ it started to rain this morning.    — I think it is just timely (及时的) rain. The weather report says the rain won’t stop _____ tomorrow evening. A. when; since             B. while; until             C. when; until           D. while; since 答案：C 要使一束平行光经过两个相同的凸透镜后，光线仍然保持平行，那么这两个凸透镜的主光轴应重合，两个透镜间的距离是 [　　] A．等于1倍焦距 B．等于2倍焦距 C．大于2倍焦距 D．可以等于任何值