我国法定计量单位中压强的单位符号是（）。
A.N
B.Pa
C.N.m
D.kgf／cm2
参考答案：B
乙肝第一次接种的年龄是（）
A.出生时
B.1个月
C.2个月
D.3个月
E.4个月
- Mr Li is said to be very patient with his students.
- Yes. But it's really a hard_______ for him to control a class of young children.
A．work
B．skill
C．task
D．experience
