对复杂，精密和表面光洁度要求高的塑件需要采用（）钢或（）钢，它们是加工性好和有极好抛光性的钢。
参考答案：Cr12MoV；4Cr5MoSiV
邮票最基本的功能是作为邮件交寄人向邮局纳付（）的凭证。
–Hi, Tom. Any idea where Jane is?
--She _______ in the classroom. I saw her there just now.
A．shall be
B．should have been
C．must be
D．might have been
