Only after liberation（解放） to be treated as human beings.
A. did they begin
B. they had begun
C. they did begin
D. has they begun
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(答案→)A. did they begin
B. they had begun
C. they did begin
D. has they begun
解析：考查倒装句。当only位于句首时，主句部分采用部分倒装结构，结合时间状语after liberation解放后，可知描述的是过去发生的动作，故助动词过去时态，选A。