A) express C) imply
B) show D) mean

A．Zoos play an important role in educating people about the conservation of wildlife and plants.
B．Zoos are a good device against species extinction.
C．Zoos valuable role in society is still an issue in debate.
D．Zoos are regularly visited by most city people.

A．zoos are different in many respects
B．many zoos serve to educate the visitors
C．some zoos act as a safety device for rare animals
D．zoos are being improved all the time