A big step   ____   to strengthen China’s health care systems since we launched an effort in 2008.

A．has been taken

B．is being taken

C．was taken

D．had been taken

以下材料中强度等级标准中需考虑具有95%强度保证率条件而确定的是( )。

A．水泥

B．建筑砂浆

C．混凝土

D．钢材