Suppose Julia and Zach are the only consumers of milk. Julia's demand for milk is defined as Q Julia 12-3P at prices below $4 and zero for prices above $4. Zach's demand for milk is defined as QdZach 10-2P at prices below $5 and zero for prices above $5. If the market price for milk is $4.50, market demand is: a) 1 units of milk. b) 10 units of milk. c) zero units of milk. d) 1.5 units of milk.