In American, a friend inviting you to a restaurant doesn't mean that he or she will pay for the meal ______ you are told ahead.

A．until

B．unless

C．although

D．if

20℃时，NaCl的溶解度为36g．现将l00g20℃时的NaCl饱和溶液加热蒸发掉部分水后又冷却到20℃，则剩下的溶液与原溶液相比（　　）

A．仍为饱和溶液

B．变成不饱和溶液

C．含溶质的质量不变

D．含溶剂的质量不变