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
C. started
D. starts
解析：考查时态：句意：杰克发了几份电子邮件后，开始做他的课题。状语从句是过去完成时，开始做课题是过去发生的，才能构成“过去的过去”的动作，所以选C。
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