When President Obama took the stage here Wednesday to address a community — and a nation — traumatized by Saturday’s shootings rampage in Tucson, Arizona, it invited comparisons to President George W. Bush’s speech to the nation after the attacks of Sept. 11, 2001, and the memorial service President Bill Clinton led after the bombing of a federal office building killed 168 people in Oklahoma City in 1995.

某混凝土拌和物1m³的质量为2400kg，砂粒为34%，拌制1m³拌和物需用含水率3%的湿砂650kg，石子含水率为2%，拌和用水量为145kg，则其水灰比为( )。

A．0.43

B．0.46

C．0.50

D．0.53