单项选择题

A Quiet student offered room in private house. Share bath and kitchen. $ 50 weekly excluding gas/electricity	B Professional couple, 3 children, 2, 4 and 6, offer single room, rent-free, to student willing to baby-sit 3 evenings weekly, occasional weekends. Live as family.
C Double room suitable 2 students sharing. Cooking facilities, share bathroom. Non-smokers only. $ 70 each weekly, excluding gas/electricity.	D Teacher going on 3-month study course abroad willing to let comfortably furnished flat in prestige block to responsible students. 2 doublebedrooms, 1 single. Use of gm’den. Rent $ 70 each, weekly, inclusive. No late parties.
INTERESTED CONTACT: Joan Benson, student accommodation officer. Room 341 Moff Building. Fri. 10:00 a. m. --5:00 p. m.

For two students ad D would be more economical than ad C because ______.