When deeply lost in thought he often was he would forget all about eating or sleeping.

问题补充：

When deeply lost in thought, he often was, he would forget all about eating or sleeping.

A. that

B. where

C. which

D. when

答案：

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。