水泥混凝土配制强度计算涉及的因素是( )。

A．混凝土设计强度等级

B．水泥强度等级

C．施工水平

D．强度保证率。

Before welding can be done in a tank that has carried petroleum products, a certificate must be obtained from（）.

A.the customs

B.the register bureau of shipping

C.the shipyard fire department

D.a certified gas chemist