Could you tell me the d ----- between the two books?

Could you tell me the d _____ between the two books?
Could you tell me the d ______ between the two books?
还有 He's speaking Italian,not Spanish.Don't you know the _____(different)?
再请说明下什么时候的 difference 后加s 什么时候不加 我知道复数时要加
可有些地方它就只跟你说 是两个或三个之间的不同 有些干脆就不说 我在网上查了一下,相同类型的题 有的人加 有的人不加 我不知道到底该怎么办.