difficult problem可数吗

　　How to improve the quality of products was a difficult problem to him.

　　如何提高产品质量对他来说可是个难题。

　　She is working at a difficult problem in mathematics.

　　她在做一道数学难题。

　　How to grow rice in a dry area is a difficult problem.

　　在干旱地区种水稻可是一个难题。

　　When you come across a difficult problem, think about it by yourself first.

　　遇到难题的时候先自己想想。

　　The student decided to work out the difficult problem by himself.

　　这个学生决心独自解决这道难题。

　　Air pollution is a difficult problem for environmental protect.

　　城市大气污染问题是环境保护的一项难题。