单选题：In most of the United States, students attend classes for 5

In most of the United States, students attend classes for 5 days each week. Schools are requiredto hold classes for about 180 days a year. Recently, schools in some areas of the country have begunexperimenting with the new teaching arrangement of holding classes for 4 days a week. Classes wouldmeet either Monday through Thursday, or Tuesday through Friday. Some schools in 12 Americanstates are now using that schedule( 日程安排).
The students still have the same number of total hours in class as before. But they attend schoolfor a longer time on each of the 4 days. In Colorado, schools on the four-day week hold classes forseven-and-one-half hours a day for 144 days. Traditional (传统的)schools meet for 6 hours a day for 180 days.
Many schools on a four-day schedule offer special programs, trips or sports meets on the fifth day. Officials say this gives students even more time in class than on the traditional schedule. Students do not have to miss school to go to the events. They can also go to doctors or deal with other important affairs on the fifth day.
The main reason for shortening the school week is that many small schools can save money by not providing meals or transportation for students on the fifth day. And they say research shows that students are not harmed by shorter week.
根据以上内容,回答题.
（　） How many teaching hours do most of the schools have in the U. S. ? A．6 hours for 4 days a week.
B．6 hours for 5 days a week.
C．7.5 hours for 4 days a week.
D．7.5 hours for 5 days a week.