以下说法符合马歇尔一勒纳条件的是( )。
A.进出口需求弹性之和小于1
B.该国货币贬值可使出口收入减少
C.该国货币升值可使出口减少
D.以上说法均不正确
参考答案:C
Mr. Wang and his students _____ a lesson now. [ ]
A. are having
B. had
C. have
星系的哈勃分类有哪些?