题目

--Why has he been staying at home these days?      --He ___________ since a month ago.    A. has been out of work            B. was out of work    C. has lost his work               D. had left from his work 答案：A 解析: 答语中有since引导的时间状语，限定了句中的动词为持续性动词。 平面与平面垂直的性质定理可简言：面面垂直，则线面垂直．两平面垂直会有许多性质，选取这条性质作为性质定理有什么意义？这条定理都有什么应用？