There is no such thing as upstair Always use upstairs.

I always use UPSTAIRS as the noun: I like the view from upstairs; upstairs it's warmer; I'm going upstairs to bed. As an adjective I usually use UPSTAIRS: the upstairs rooms are nicer; the upstairs view is better. I might occasionally use UPSTAIR as the adjective: the upstair view is better. It's possible to use upstair in this adjectival role. But not very common. As you can use UPSTAIRS as the noun and the adjective it's easiest to use just UPSTAIRS!

Thanks,this question is the choice between upstair and upstairs. I choose "upstair "is wrong,but "upstair" is the adjective on line ,too. I'm confused .