While we appoint an agent, it is usual to make the appointment ( ).

A．in writing

B．on paper

C．with writing

D．write down

某住宅楼工程构造柱用碎石混凝土，设计强度等级为C20，已知混凝土强度标准差为4.0MPa，则混凝土的配制强度为（）MPa。

A.36.58

B.36.85

C.26.58

D.26.85