题目内容：
根据以下内容，回答题。Passage Four
During the summer session there will be a revised schedule of services for the university community. Specific changes for intercampus bus services, the cafeteria, and summer hours for the infirmary (医务室) and recreational and athletic facilities will be posted on the bulletin board outside of the cafeteria. Weekly movie and concert schedules which are in the process of being arranged will be posted each Wednesday outside of the cafeteria.
Intercampus buses will leave the main hall every hour on the half hour and make all of the regular stops on their route around the campus. The cafeteria will serve breakfast, lunch, and early dinner from 7 a. m. to 7 p. m. during the week and from noon to 7 p. m. on weekends. The library will maintain regular hours during the week, but shorter hours on Saturdays and Sundays. The weekend hours are from noon to 7 p. m..
All students who want to use the library borrowing services and the recreational, athletic,and entertainment facilities must have valid summer identification cards. This announcement will also appear in the next issue of the student newspaper.
Which of the following is the main purpose of this announcement? A．To tell campus personnel of the new library services.
B．To announce the new movies on campus this summer.
C．To notify university people of important schedule changes.
D．To remind students to validate (使生效) their identification cards.
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