The Russian rocket designer has died. For many years, he ______ as an important assistant to
问题描述：
The Russian rocket designer has died. For many years, he ______ as an important assistant to
the father of Soviet space program.
A. serves
B. was serving
C. served
D. has served
为什么选C
答
C
死前,他一直以一种助手身份服务,工作.过去时