When deeply lost in thought, he often was, he would forget all about eating or sleeping.
A. that
B. where
C. which
D. when
网友回答
A. that
B. where
(答案→)C. which
D. when
解析：
试题分析:考查定语从句。由结构判断此处是定语从句，从句缺少表语，用关系代词，故排除B、D选项， 而that不引导非限制性定语从句，所以答案选C。句意：当他陷入沉思中时，他总会忘记吃饭、睡觉，这种情况时常发生。