是in their opinion还是in their opnions

是in their opinion还是in their opnions in their opinion还是 in their opinions? 为什么in their opinion中的opinion不加s? in their opinion opinion后是否加s in their opinion 中的 opinion有复数么? in their lives还是in their life In my opinion,the students had better finish their college In my opinion,their friendship has survived the passage time people is limited for expression their opinion in some count in their life还是in their lives?这两个短语哪个是正确的, 是 the change in their numbers 还是 the changes in their number make friends in their ages还是age