Mary took his dictionary but he didn't know that (改为同义句)

问题描述:

Mary took his dictionary but he didn't know that (改为同义句)
Mary took his dictionary _____ ____.

Mary took his dictionary but he didn't know that.
= Mary took his dictionary (in secret).
注:in secret意思是“暗地里/偷偷地”,也就是说“他并不知道这件事”.