Yesterday it was Sharon's birthday, but she had no special plans, so she decided to go home after work.She left the office arid while she was walking down the street，she was1her friends.They usually remembered her birthday but they had forgotten it for the first time this year.“I think that's2because all of them are busy at work.I guess they will call me tomorrow as it is Saturday, ”she said to herself.Then she3that she was hungry, so she bought some food in a supermarket.And when she came out of the supermarket，she saw an old woman selling4.She bought some roses(玫瑰)from her and left.

    After she got home，she took a shower.And then she had some food.“Now I feel much happier,”she said.“If5remembers my birthday, I will celebrate it by myself.”Suddenly the doorbell (门铃)rang.It was 11：00 p.m.She was surprised 6her husband was on a business trip.“7could it be?”She thought and went to open the door.

    “Happy birthday! ”A group of her friends and her husband were there! Also their hands were full of colorful packages(包裹).She couldn't8her eyes. She had everybody9her and many nice gifts.It was the most10moment (时刻)of her life.“I'm really happy to have such thoughtful friends like you.Thank you all!”Sharon said excitedly.

下列对动物行为研究的案例中，纯粹属于观察法的是（　　）