3、Modern zoos are very different from zoos that were built fifty years ago. At that time, zoos were places _ _ 21 _ _ people could go to see animals from many parts of the world. The animals lived in cages that were made _ _ 22 _ _ 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 but natural. _ _ 23 _ _ 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 given more _ _ 24 _ _ 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 _ _ 25 _ _ the areas that animals live in.