一个圆锥滚子轴承内径是90mm，其代号可能是（）
A.71918
B.31918
C.71990
D.31990
参考答案：B
It was unusual for there （）so few orders in a month.
A. to be
B. be
C. being
D. are
A．O3
B．CO
C．NO
D．NO2
