The demand equation for a firm's product is given by the equation ln Q = 100 - 0.25 ln P - 0.5 ln M + 0.5 ln A + 10 ln PY, where Q = quantity, P = price, M = income, A = advertising expenditures, and PY = price of a related good. Based on this demand equation, this good is:
a. inferior.
b. normal.
c. a necessity.
d. a luxury.
e. None of the above.