A paint store sells two brands of latex paint. An analysis of total sales figures indicates that the demand D (in gallons per month) is modeled by D(p, q) = 210 - 121ĀĀ(18q), where p and q are the prices of the first and second brands, respectively. A separate study indicates that in n months from now, the first brand will cost p = 4 + 0.1n dollars per gallon and the second brand will cost q = 5 + 0.1n dollars per gallon. Calculate the rate at which the demand D will be changing in n months from now. Justify your answer.