The Key Difficulty in Answering the Question: "Would You Be

The key difficulty in answering the question: "Would you be better off financing your new home with a 15-year mortgage at 9% or by borrowing for five years at 8% and refinancing thereafter?" is that

A) housing prices are very erratic.
B) the tax deductibility of mortgage interest payments has changed over time.
C) dollars paid in different periods are not in the same units.
D) 15-year mortgages are fixed-payment loans while 5-year mortgages are simple loans.