When I was looking for a Christmas present

问题：

When I was looking for a Christmas present for my daughter in a toy store, a nicely dressed little girl, with some money in her little hand, was looking at some beautiful dolls. When she saw a doll she liked, she would ask her father if she had enough money. He usually said yes.
At the same time, a boy, with old and small clothes, was looking at some video games. He, too, had money in his hand, but it looked no more than five dollars. Each time he picked up one of the video games and looked at his father, he shook his head.
The little girl had chosen her doll, a very beautiful one. However, she noticed the boy and his father. She saw the boy give up a video game with disappointment and walk to another corner of the store.
The little girl put her doll back to the shelf and ran over to the video game. After she talked to her father, she paid for the video game and whispered(耳语) to the shop assistant.
So the boy got the video game that he wanted for free—he was told it was a prize from the store. He smiled happily, although he felt it was so incredible. The girl saw all this happen. She smiled, too.
When I walked out of the store to my car, I heard the father ask his daughter why she had done that. I would never forget their short talk. " Daddy, didn’t Grandma want me to buy something that would make me happy"
He said, "Of course, she did. "
"Well, I just did!" With that, the little girl started skipping towards their car happily.

What does the sentence "Well, I just did!" mean

A．I just did something for the boy and he would be happy.
B．I just bought a nice doll for myself and I would be happy.
C．I just bought a present for Grandma and she would be happy.
D．I just did something for the boy and it would make me happy.