问题补充：
the police thought he was the most likely one，since they had no exact proof about it，they could not arrest him.
A. Although
B. As long as
C. If only
D. As soon as
答案：
(答案→)A. Although
B. As long as
C. If only
D. As soon as
解析：根据句意，引导让步状语从句。故选A。
