Let me tell you my pen friend Ivan. He is 12 years old. He likes playing football. He usually play football with his friends at weekends.
Ivan lives with his father and mother in a city in France. They have a dream home with five rooms, a swimming pool and a garden. They often have a party in the garden at weekends. They like to help their neighbours. His home is not near his school, so Ivan takes a bus for an hour and then walks for 10 minutes to school. There is a Basketball Club near his school. He likes play basketball in the club.
   Ivan's father is a worker. He is busy. Ivan's mother is a doctor. She is nice to the patients. Ivan wants to be a teacher when he grows up. I want to be a teacher, too. Every month Ivan writes two letters to me. He'll come to Suqian next year. We'll meet then.

已知F₁，F₂为椭圆C的两个焦点，P为C上一点，若|PF₁|，|F₁F₂|，|PF₂|成等差数列，则C的离心率为____．

看书一小时后，向远处眺望一会儿，可以预防近视眼，主要原因是 (  )