题目

 Though many students fail to pass the National College Entrance Examination every year, they will have learned many ________ lessons such as hard work, devotion, communication etc. during their high school.     A. reasonable             B. popular         C. amazing        D. valuable 答案：D作变速直线运动的物体，若前一半时间的平均速度为8m/s，后一半时间的平均速度是16m/s，则全程的平均速度是（　　）A．14 m/sB．10 m/sC．11 m/sD．12 m/s