The two friends ______on the Internet for three hours, and they would go on till the next morning.

A．have been chatting

B．had been chatting

C．were chatting

D．had chatted

如图是硝酸钾的溶解度曲线图，某同学在20℃时进行了如下实验，得到相应的溶液①～⑤，下列说法正确的是（　　）

A．保持温度不变，若在④中加入4.5KNO₃固体可得到饱和溶液

B．②③⑤中溶质的质量分数相等

C．①③④均为不饱和溶液

D．④→⑤可析出53.9gKNO₃晶体