The General Linear Demand Function Below Is Used to Answer $Q _ { d } = a + b P + c M + d P _ { R }$

The general linear demand function below is used to answer questions :
$Q _ { d } = a + b P + c M + d P _ { R }$
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.