1 Traditional, barred cages are almost absent from modern zoos. 2 Instead, animals in modern zoos live in surroundings similar to their natural environment in the wild. 3 Artificial storms are designed, and deserts are constructed, to meet the needs of a wide variety of beasts, fish, and fowl. 4 Fish exhibits are particularly popular at zoos. 5 Tape recordings of familiar noises are used to soothe animals and help them adjust to new homes. 6 Gorillas, for example, often become sluggish in captivity. 7 They become more active when exposed to recordings of sounds that they would encounter in African mountain forests.

Which sentence could you remove from the paragraph to improve its unity?

A. 4
B. 2
C. 5
D. 3