salad是可数名词吗为什么有时候+s

　　例句：

　　The salad had lost its crispness.

　　这色拉已经不脆了。

　　Can I help you to some more salad?

　　再给你来点色拉好吗？

　　I think I'll go for the fruit salad.

　　我想要水果色拉。