Although the police thought he was the most likely one they had no exact proof about it

问题补充：

Although the police thought he was the most likely one, they had no exact proof about it, they could not arrest him.

A. when

B. until

C. before

D. since

答案：

A. when

B. until

C. before

(答案→)D. since

解析：考查连词. A.当……时候B.直到…… C.在……以前 D.因为；由于；自……以来。句意：尽管警察认为他是最可能的一个，由于他们没有确切的证据，他们也不能逮捕他。选D。