历法所依据的天文周期有（）
A.恒星年
B.回归年
C.恒星月
D.朔望月
参考答案：B, D
There is an hour to go. Could we sit somewhere to have a drink?（）
A.Have a try.
B.Sure.
C.Don‘t go, please.
D.No, thanks.
替代强化
