On July 31, Agnes moved out of the house she has owned for five years because her job transferred her to another state. She rented her house to a college student at a fair rental price from August 15 until December 15 when the semester ended. When Agnes returned to her hometown for a week at Christmas she stayed at her house because the college student had already moved out. How many days must be counted as days of personal use?

a) 0 b) 7 c) 16 d) 30