No one knows 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：词义辨析。came into power掌权，上台；came into being 形成；came true 实现；came out出来，出版。句意：没有人知道这样的习俗最初是怎样形成的。所以选B。