The police had been working on the case for over three months without much result, but they refused to give up.They （） it till they finally arrested the ring-leader.

The police had been working on the case for over three months without much result, but they refused to give up.They （） it till they finally arrested the ring-leader.
A.would aim at
B.would live at
C.would keep at
D.would stay at
正确答案：C