The problem is so difficult that few students can solve them.这句话有没有语病?我觉得应该用The problems is so difficult that few students can solve them.或者The problem is so difficult that few students can solve it.到底怎么样才对

The problem is so difficult that few students can solve them.
这句话有没有语病?我觉得应该用
The problems is so difficult that few students can solve them.
或者
The problem is so difficult that few students can solve it.
到底怎么样才对