The function f(x)=5000(0.98)^0.3x represents the number of white-blood cells, per cubic millimeter, in a patient x days after beginning treatment for a virus. What is the average decrease per day in white-blood cells per cubic millimeter between days 1 and 5?
a. -29.75mm^3/day
b. -34.75mm^3/day
c. -282.47mm^3/day
d. -353.08mm^3/day