人的一生有两副牙是（）
A.前牙、后牙
B.切牙、磨牙
C.大牙、小牙
D.尖牙、磨牙
E.乳牙、恒牙
参考答案：E
—Who is making much noise?
—It ____ be Timmy. He is sleeping now. [ ]
A. can't
B. may not
C. mustn't
D. couldn't
放牧对草地有哪些积极影响和消极影响？
