I learned a lot ___ in the countryside.

题目

I learned a lot ___ in the countryside.
A.working B.while working C.when worked D.to work
请问为什么

答案

省略,当主句从句的主语相同的时候,
从句中,可以省略主语和助动词
完整的：i learned a lot while I was working in the countryside.
省略,就是 ...while working ...