单选题 .  "Sesame Street" (芝麻街) has been called "the longest street in the world". That is because the television program by that name can now be seen in so many parts of the world. That program became one of America's exports soon after it went on the air in the New York in 1969.
    In the United States more than six million children watch the program regularly. The viewers include more than half of the nation's pre-school children. Although some educators object to certain elements in the program, parents praise it highly. Many teachers also consider it a great help, though some teachers find that problems arise when first graders who have learned from "Sesame Street" are in the same class with children who have not watched the program.
    The program uses songs, stories, jokes and pictures to give children a basic understanding of numbers, letters and human relationships. Tests have shown that children have benefited from watching "Sesame Street". Those who watch five times a week learn more than the occasional viewers. In the United States the program is shown at different times during the week in order to increase the number of children who can watch it regularly.
    Why has "Sesame Street" been so much more successful than other children's shows? Many reasons have been suggested. People mention the educational theories of its creators, the support by the government and private businesses, and the skillful use of a variety of TV tricks. Perhaps an equally important reason is that mothers watch "Sesame Street" along with their children. This is partly because famous adult stars often appear on "Sesame Street". But the best reason for the success of the program may be that it makes every child watching it feel able to learn. The child finds himself learning, and he wants to learn more.1.  Why has "Sesame Street" been called "the longest street in the world"? ______