Let's __who lost the key.A.look for B.find C.find out

c
A是强调寻找过程
B找
C找出，强调结果

C

选C

选C
这是定语从句用（who）来修饰
前面要是一个完整的句子

选择find
find vt.找到, 发现, 感到, 查明, 得到, ----
用在句子中强调的是找的过程 例如
To come upon after a search:
找到：通过寻找遇到：
found the hidden leak in the pipe.
在烟斗上找到一个隐藏的缺口
To discover or ascertain through observation, experience, or study:
发现，确定：通过观察、经验或研究发现或查明：
found a solution; find the product of two numbers; found that it didn't really matter.
发现解决办法；发现两个数的结果；发现无关紧要
To perceive to be, after experience or consideration:
认为：经验或考虑之后认知：
found the gadget surprisingly useful; found the book entertaining.
发觉这个妙计非常有用；认为这本书很有趣
find out 强调寻找后的结果
To ascertain (something), as through examination or inquiry:
查明：如通过观察或询问查明（某事）：
I found out the phone number by looking it up. If you're not sure, find out.
通过查找我查到了电话号码。如果你不肯定，查一查
To detect the true nature or character of; expose:
发现：