—How often _____ she exercise?  

—Three times a week.   [ ]

A. do

B. does

C. doing

D. did

关于地球大气各层热量来源的正确叙述（）

A.高层大气热量主要来自平流层顶

B.平流层热量主要来自对流层顶

C.对流层大气热量主要来自地面辐射

D.对流层热量主要来自太阳辐射