When I was seven years old my morn was diagnosed(诊断) with cancer. The doctors told us my morn might be saved with surgery(手术). But it could also kill her. She chose to have it. The day before the surgery I was off school, and my mom planned the best day of my life, everything I loved at seven and everything that would put a smile on my lace. The day began with her waking me up saying, "Kate, I have a surprise for you. Come and see." The surprise was a doll I had wanted for the longest time. Throughout the day, she told my everything that she thought I would need to know to grow up and be a good person; she told me to be the best I could be and that I would always make her proud. We had a picnic in the park, and it was so cold that we moved to the car. The day was filled with laughter. For the first time in a long time I could see she was really happy. I would never forget her smile, or the way her eyes shone as if we were the same age. It was the best day of my life, and I will never forget the conversations we shared. My mother managed to live through the surgery. Now when I look back, I realize that the best day of my life could be her last and this could be the last day I would remember with her, the last one we shared. I also understand how unselfish a mother is. Why was the writer’s mother happy that day

A. Because she left her child a special memory.
Because she realized her child had grown up.
C. Because she was proud of what her child had done.
D. Because she expected the surgery to be successful.