在西部大开发中，要对部分耕地“退耕还草、还林”，在这个过程中农田的演替属于 

根据短文理解，完成下列各题。

    Helen and Linda are good friends. They are in the same class. They go to school five days a week. And they have seven classes a day. After school they play volleyball.

    The two girls' mothers are friends, too. They like sports. So they can play sports with the two girls on the weekend（在周末）. Linda's mother likes tennis and Helen's mother likes tennis, too. Can the two girls play tennis? Sure, they play it well.

    The four friends always play tennis in the morning, and watch TV in the afternoon. It's relaxing! The two girls like eating fruits. They like apples, strawberries and oranges. They think fruits are good and healthy. After playing tennis, their mothers always make (做) some fruit salad first. What good friends they are!