A Large Mixing Tank Initially Contains 1000 Gallons of Water $A ( t )$

A large mixing tank initially contains 1000 gallons of water in which 40 pounds of salt have been dissolved. Another brine solution is pumped into the tank at the rate of 5 gallons per minute, and the resulting mixture is pumped out at the same rate. The concentration of the incoming brine solution is 3 pounds of salt per gallon. If $A ( t )$ represents the amount of salt in the tank at time t, the correct differential equation for A is

A) $\frac { d A } { d t } = 3 - .005 A$
B) $\frac { d A } { d t } = 5 - .05 A$
C) $\frac { d A } { d t } = 15 - .005 A$
D) $\frac { d A } { d t } = 3 - .05 A$
E) $\frac { d A } { d t } = 15 + .05 A$