Mary had to go to a meeting,so she left her children________at home.[ ]

A．playing  

B．played

C．to play  

D．having played

某工厂拟选用下列物质处理含硫酸的废水，想花最少的钱中和等量的废水应选择（　　）

A．氯化钠

B．碳酸钙

C．熟石灰

D．氢氧化钠