Twenty-five years ago the number of the museums in the United States and Canada was only about ______.

A．the scientific rules are important
B．people learn best when they look at something
C．visitors prefer to learn from museums
D．people become more interested if they take part in

A．people should go to a museum to learn something
B．people in the United States don't have much free time
C．people are enjoying themselves fully in modern museums
D．in the United States there aren't enough museums