Yesterday when my son and I were eating dinner, I saw a policewoman walking past our house. A woman with a1look was following her.
      I opened the 2 , went out and asked if everything was OK. The policewoman said that the woman's3 had gone missing(失踪) an hour earlier. I said that we hadn't seen the girl, but wecould4.Iexplained to my son that we couldn't finish our5 right then. He said,” Be a kind person, Dad. "We got in our car and6 the girl all over the town. Along the way,a group of chil­dren were playing.  7they heard what had happened, they said they would help8 . They rode their bikes and joined us. 9 the little girl was found safe a few miles away. I was so 10to see many people help a stranger. It is a nice world.

下图为某特大城市及其周边地区各类土地付地租能力与该地区地形剖面示意图。读图，图中b1、b2功能区相同，但级别不同，则在b2地区发展该功能区的原因是（    ）