中国当前正肩负着提高____、增强____和提高____的历史任务。
Ann is ____ (young). Lucy is ____ (young) than Ann. Lily is ____ (young) than Lucy. So Ann is the ____ and Lily is the ____.
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____.