No one knows for sure when such a custom first .
A. came into power
B. came into being
C. came true
D. came out
网友回答
A. came into power
(答案→)B. came into being
C. came true
D. came out
解析：考查词组辨析：A. came into power开始执政，B. came into being开始形成C. came true成为现实， D. came out出版，发行，句意是：没有人知道这种风俗是什么时候形成的。选B。