题目内容：

求原因,谢谢(为什么A不对)
Once they watched a whale -----the depth of 400 metres underwater
a in
b of
c at
d on

优质解答

选 C 表示 具体的深度,用介词 at at the depth of 意思是“在.的深度”
翻译：曾经他们在水下400米处看见了鲸.