题目

We live in a beautiful cottage with a yard, which ______ 20 feet from side to side. A. measures    B. is measured     C. measured            D. has been measured 答案：A7．太阳、月亮、萤火虫，其中月亮不是光源；光在真空中的传播速度约是3×108m/s；光在同种均匀介质中沿直线传播．