During the years _______, he worked as a doctor in the hospital.

A. followed B. to follow C. following D. that followed
答案D，请解释。

D. that followed 引导的是一定语从句，引导词that代替先行词years.且在从句中作主语。也相当于下面的短语during the following years.

你好！ that 不可省，因为 the years 在从句中that作主语。 希望对你有所帮助，望采纳。