单项选择题

During the past 30 years or so, television has become a major agent of socialization(社交), often competing with parents and teachers. It occupies more of the working hours of American children, and so it is called "a member of the family". School age children watch television between seventeen and thirty hours a week. For preschool children it is often as high as fifty-four hours a week.
By the time the youngster graduates from high school, he or she will have spentroughly 11,000 hours in school compared to more than 22,000 hours in front of television. Children are not just watching children’s programs. On the contrary, the largest part of their viewing takes place between 8 and 11 p.m. Monday throughSaturday and between and 11 p.m. on Sunday.
However, 60% of mothers said they placed no "restrictions" on the amount oftime; they permitted their children to watch television. Teachers and parents have become increasingly concerned about the effects of television on school performance. Many teachers have reported more and more incidences(发生) of fatigues, tension, and bad behaviors. To minimize the harmful effects of television, parents and teachers at Kimberton Farms School in Phoenixville, Pennsylvania, have been following written guidelines for five years which include no television at all for children through the firstgrade.
Children in second grade through high school are encouraged to watch notelevision on school nights and limit viewing to a total of three to four hours onweekends. As many parents reported, the result was obvious. Three days after theyturned off the sets, the children could concentrate better, and were more able tofollow directions and get along with their neighbors. Recently, the National ParentTeacher Association has been working hard in offering advice on choosing programs, setting time limitations, and helping parents and children develop critical attitudes.

The main point of this selection is that（）

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