She told me that she has already gone to the United States four times before she attended that conference.

A．had already been

B．has already been

C．already went

D．was already going

某化学兴趣小组设计四种实验方案来证明炭粉中是否含有氧化铜.其中可行的是：

A．将固体高温加热，看是否有红色固体生成

B．在样品中加稀硫酸，充分反应后，观察是否有固体剩余

C．在样品中加水，是否有蓝色沉淀

D．在样品中加硫酸，充分反应后，溶液是否呈蓝色