Sustainable cities often plan for "Green Belts"—areas of parks and trees surrounding or inside the city. What is a primary "Green Warrior" reason for this?
(A) To provide a place for people to take beautiful photos.
(B) To act as "lungs" for the city by filtering air pollution and providing oxygen.
(C) To make sure the city doesn't grow too large.
(D) To provide wood for building more houses.