下图是某地地形图，MN、PQ是地形剖面线。①、②是水库，若从中选择一个作为自来水厂的水源地，其条件是自流引水且工程建设费用最小。完成下列小题。

撒哈拉以南的非洲地区经济特征是（   ）

    There is a new park near Andy's home. It's fine today. Andy and his family are in the park now. On their left, there is a café. On their right, there is a big lake. There are many flowers and trees around the lake. There's a small hill behind the lake. Near the lake, there are two signs. One says, “Don't swim in the lake!” The other says, “No fishing!” But you can go boating on the lake. There is a beautiful garden in the middle of the park. There are green grass and beautiful flowers in it. There are some small shops between the lake and the garden. The park is so beautiful. Andy and his family like it very much.