单项选择题

《混凝土结构设计规范》规定预应力混凝土构件的混凝土强度等级不应低于（）

A．C20 ;
B．C30 ;
C．C40 ;
D．C50 ;

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单项选择题

A hump yard is a railway yard found at some freight train stations, used to separate railway cars onto one of several tracks.

A．对
B．错

单项选择题

Why does the CEO invite Chris and his son to watch football with him?
A、He is moved by Chris’s sincere attitude.
B、He likes Chris’s son.
C、He has no companion to the football game.
D、He wants to know more about Chris’s family.