There______ a lot of snow in our city last year. [ ]
A. is
B. are
C. was
D. were
答案:C
-What's that? -It's _apple.
A.a
B.an
C.the
D./
______是大气中的一种痕量气体,基本上都集中在平流层底部和对流层中,距地面20~25km,浓度不超过10kg/km3,但是在保护地球生态环境中起着重要的作用。