Nobody could explain the _______ of this commonly-used expressio

Nobody could explain the _______ of this commonly-used expression.
A、beginning
B、starting
C、cause
D、origin
【正确答案】：D
【题目解析】：beginning开始；starting开始；cause原因；事业；目标；origin起源；原点；句意：没有人能解释这个常用短语的由来。