一道非谓语动词的题The secretary worked late at night,( )a long speech

一道非谓语动词的题
The secretary worked late at night,( )a long speech for the president.
A.to prepare
B.preparing
C.prepared
D.prepare
选什么?为什么不能选其他选项?
为什么a是将来时
to do 不是表目的么？为什么不能选？