题干
．I have to see the doctor because I________a lot lately．
A：cough
B：coughed
C：have been coughing
D：had coughed
答案(点此获取答案解析）
C
A：cough
B：coughed
C：have been coughing
D：had coughed
，不考虑卫星质量的变化，则变轨前后卫星的
