I had been working on math for the whole afternoon and the numbers before my eyes.
A. swim
B. swum
C. swam
D. had swum
网友回答
A. swim
B. swum
(答案→)C. swam
D. had swum
解析：主句用过去完成进行时，所以后半句用一般过去时，只有这样才能具备“过去的过去”的条件。句意：我一整个下午都在做数学题，那些数字在我的眼前晕眩。
【考点定位】考查动词时态。