He stayed up late last night,___ a speech for his boss.A.to prepare B.preparing

问题描述:

He stayed up late last night,___ a speech for his boss.A.to prepare B.preparing
为什么不能选a表目的?

选A的情况下是不需要逗号的,表示目的.现在有逗号只能用现在分词做伴随状语.