Which of the following uses of a dwelling unit does not count as personal use?

a. The taxpayer's parents use the unit for two weeks and pay a fair rental price.

b. The dwelling is rented to a friend for less than a fair rental price.

c. The taxpayer stays at the dwelling while working full-time to replace the roof.

d. All of the above count as personal use.

e. None of the above count as personal use.