Which investment produces a $5 hourly profit for a candy shop earning $1 profit per pound of candy?

a. worker at $10 per hour, producing eight pounds of candy per hour
b. worker at $12 per hour, producing 16 pounds of candy per hour
c. machine with $5 per hour operating cost, producing 10 pounds of candy per hour
d. machine with $8 per hour operating cost, producing 14 pounds of candy per hour