问题补充：
After Jack had sent some e-mails, he working on his project.
A. had started
B. has started
C. started
D. starts
答案：
A. had started
B. has started
(答案→)C. started
D. starts
解析：考查动词时态。根据前半句的谓语动词用了过去完成时，可知后半句用一般过去时，这样才能符合“过去的过去”这一条件。句意：Jack发了几份电子邮件之后，他才开始研究他的项目。
