Hello! I'm Linda. I have a good e-friend. Her name is Zhang Yun. Her English name is Betty. She is a Chinese girl, but she lives in the UK with her parents. Her father is a doctor and her mother is a teacher. She is ten years old. She call me big sister. She is good at English. She likes Chinese and I also like Chinese. She likes singing and playing basketball. This is my good e-friend. Do you want to know more?

如图，在△ABD中，C是BD上一点，若E、F分别是AC、AB的中点，△DEF的面积为4.5，则△ABC的面积为 ____．

我国古代的四大发明出现在隋唐时期的是（  ）