Rosa replaced a broken heating and air conditioning unit in one of her rental houses. How will this expense be reported on schedule E ? ( she will not make the safe harbor election for small taxpayers)

a. As a repair, deducted as a current expense

b. As an improvement, depreciated over 15 years

c. Asan improvement, depreciated over 27 1/2 years

d. As an improvement, depreciated over 31 1/2 years.