Modern zoos are very different from zoos that were built fifty years ago. At that time, zoos were places ___1___ people could go to see animals from many parts of the world. The animals lived in cages that were made ___2___ concrete with iron bars, cages that were easy to keep clean. Unfortunately for the animals, the cages were small and impossible to hide in. The zoo environment was anything ___3___ natural. Although the zoo keepers took good care of the animals and fed them well, many of the animals did not thrive; they behaved in strange ways, and they often became ill. In modern zoos, people can see animals in more natural habitats. The animals are ___4___ more freedom in large areas so that they can live more comfortably as they would in nature. Even the appearance of zoos has changed, trees and grass grow in the cages, and streams of water flow ___5___ the areas that animals live in. 根据短文内容，在A、B、C、D、E五个选项中，选出最佳答案填入文中【4】处。（）
A of B but C where D over E given