they are good friend 朋友加s 还是不加?

they are good friend 朋友加s 还是不加? they are good friends of ours.our为什么加s They are (Jill's good friend.)对括号部分提问 They are books.book为什么加s let,s are good friend还是let,s be good friend 为什么what makes good friend?make要加S? what's the price of 后加复数 回答用it's 还是they are the best things in life are happy中的thing 加还是不加S,为什么? They are proud of their player.为什么这里的player不加s? they are in different size(s)要加S吗 ( )my good friend,Kate and Mary.选they are还是these are? 为什么 they all are friend 还是 they are all friend