A small fishing boat's outboard motor requires a mixture of gasoline and oil for fuel. The mixture called for in the owner's manual is 128 ounces of oil for every 50 gallons of gas. If the boat's gas tank holds approximately 6 gallons of gas, approximately how much oil must be added to the gasoline?
a. 3 ounces
b. 9 ounces
c. 15 ounces
d. 24 ounces