Use the Following General Linear Demand Function Below: Where

Use the following general linear demand function below: where Q_d = quantity demanded,P = the price of the good,M = income, = the price of a good related in consumption.The law of demand requires that

A) a < 0.
B) b < 0.
C) P < 0.
D) a < 0 and b < 0.
E) b < 0 and P < 0.