ours is a socialist country 不明白这里为什么要用ours,不应该是our么?

ours is a socialist country 不明白这里为什么要用ours,不应该是our么? a friend of ours写成our a friend可以吗`ours换成us可以吗/为什么? jackie is our favorite actor为什么不是ours 用our和ours造句 （）school is big,but（）is small.A.Our；theirB.Our；theirsC.Ours； China is a ______ socialist country. 2.He is a good friend of ( ).A.our B.our's C.ours We,Us,Our,Ours要怎么区分? His answer is different ( )ours a. -------school is much bigger than-----.A.Our;their.B. Ours;t mr song is ( )headmaster.A,we B,our C,ours D,us look,that is ( )music club.A,we B,our C,ours