Definition: causing something, or responsible for something Level: C1
他突然改变主意的背后动机是什么？
What was the motivation ___behind his sudden change of heart?
他辞职背后的真正原因仍然是个谜。
The real reason ___behind his resignation remains a mystery.
贪婪是这次公司丑闻背后的主要驱动力。
Greed was the main force ___behind the corporate scandal.
