They thought about it for a long time but still could not a solution.A. come overB. come

问题补充：

They thought about it for a long time, but still could not a solution.

A. come over

B. come along with

C. come out

D. come up with

答案：

A. come over

B. come along with

C. come out

(答案→)D. come up with

解析：A过来,(从远处)来到;横过;越过B随同;和……一起来 C出现,出版,结果是,D提出，想出办法，建议。句意：他们想了很长时间，但是仍然不能想出解决方法。