My brother has _____ in Japan. [ ]
A. pen pal
B. a pen pal
C. two pen pal
D. a pen pals
答案:B
粉尘爆炸与气体爆炸相比,所需的点火能()。
A.大
B.小
C.相等
何谓季节性繁殖动物?何谓常年繁殖动物?举例说明。