The demand, D, for a new rollerball pen is given by D = 0.007p" -0.9p + 140p, where p is the price in dollars. a) Find the rate of change of quantity with respect to price, dD/dp b) How many units will consumers want to buy when the price is $25 per unit? c) Find the rate of change at p = 25, and interpret this result. d)Would you expect dD/dp to be positive or negative