根据短文内容，回答下列问题。
   Linda and Mary are good friends. Linda is thirteen. She comes from the U.S.A. Mary is twelve. She's from London(伦敦). They study in the same school, but in different classes. Linda's father teaches English in a middle school. Her mother drives a car. Mary's mother works in an office. Her father is a doctor. Linda can speak English, French(法语) and a little Chinese. Linda's mother likes cooking. She likes eating rice. But Linda' s father doesn't. He likes drinking milk and eating bread. Mary's mother doesn't like cooking. But she likes doing some shopping. She can make nice skirts and dresses. Mary's father likes doing housework(家务).

某干旱半干旱地区人口持续增长。下图示意该地区25人/平方千米等人口密度线的变化。完成下列各题。