You ________ sleeping on the train. You were very loud![ ]
A. are
B. were
C. was
答案:B
群落演替
随着大气臭氧层变薄,照到地面的紫外线增强,使全球皮肤癌患者增加。其致癌因子主要属于()
A.物理致癌因子
B.化学致癌因子
C.生物致癌因子
D.以上三者都是