He has worked the maths problem for nearly an hour but he simply can’t work it.On ;atB. on

问题补充：

He has worked the maths problem for nearly an hour but he simply can’t work it.

On ;atB. on ;out C. at ;up D. for ;out

答案：

解析：考查介词。work on从事，忙于；work out解决。句意：他已经做那条数学问题接近1个小时了，但他还没有做出来。