题干
---May I go and play with Tom this afternoon, mum?
---No．You can’t go out ______ your homework is being done．
---No．You can’t go out ______ your homework is being done．
A：until
B：after
C：before
D：a
答案(点此获取答案解析）
D
＝
，则cos 
＝( )．