断奶前仔猪死亡的主要原因有()、()、()和()。
参考答案:寒冷;饥饿;挤压;疾病
— You speak such good English.Have you been abroad before?
— Yes.I ________ in Britain four years ago.[ ]
A. studied
B. had studied
C. was studying
D. have studied
在SQL语言中授权的操作是通过( )语句实现的。
A.REVOKE
B.INSERT
C.CREATE
D.GRANT