Use the Following General Linear Demand Function Below $Q _ { d } = a + b P + c M + d P _ { R }$

Use the following general linear demand function below: $Q _ { d } = a + b P + c M + d P _ { R }$ where Q_d = quantity demanded,P = the price of the good,M = income, $P _ { R }$ = 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.