A.watched/ would ringB.would watch/ had rung

C.was watching/ rangD.had been watched/ was ringing

【答案】C

【解析】

句意：当我看电视时，门铃响了。while引导的时间状语从句，通常为进行时态，其动词部分结构为be + doing，选项C符合题意，故选C。