A 50-L tank initially contains 4 g of salt dissolved in water. Brine with a concentration of 0.5 g/L flows into the tank at 3 L/min, and the well-stirred mixture flows out at the same rate so that the volume remains constant. Which differential equation best models the rate of change dtdQ of the amount of salt Q(t) (in grams) in the tank?
a) 1.5−50Q
b) −1.5+503Q
c) 1.5−503Q
d) 0.5−503Q