We say "U.S. citizen", but why can't we say "China citizen"? Or can we?
Is the word "U.S." an adjective or a noun modifier in this case? It seems to me that it is an adjective that goes before citizen, because we say "Russian citizen", "Chinese citizen", etc. But if it's a noun modifier, then why can't "China" be a noun modifier, too? Although "China citizen" sounds wrong to me, "(People's) Republic of China citizen" doesn't sound as bad. I know I could always write "citizen of China" and in general, "citizen of [country]", but I'm just curious to know.