-What's up? You look worried.
Well,I on the problem for 5 hours but I haven't got a single clue.
A. have worked
B. Worked
C. will work
D. have been working
网友回答
A. have worked
B. Worked
C. will work
(答案→)D. have been working
解析：考查时态。从时间状语 for 5 hours，可排除BC，然后根据 but I haven't got a single clue，说明work这个动作还要持续下去，故用现在完成进行时。句意：——怎么啦？你看起来很焦急。——唔，我花了5个小时在这个问题上但现在还没有什么头绪。