—Why do you look so puzzled?
—Oh，I over a most difficult maths problem.
A. thought
B. have thought
C. am thinking
D. think
网友回答
A. thought
B. have thought
(答案→)C. am thinking
D. think
解析：考查时态。句意：——为什么你看上去这么困惑？——哦，我正在思考一道数学难题。根据问话者说的“look so puzzled”可知，“思考难题”这一动作正在进行当中，故选C。