Joanna and David are farmers who produce beef and corn. In a year, Joanna can produce 10 tons of beef or 60 bushels of corn, while David can produce 10 tons of beef or 50 bushels of corn. To maximize their total output of beef and corn,
a. only Joanna produces because she has an absolute advantage over David
b. David produces beef and Joanna produces corn
c. Joanna produces beef and David produces corn
d. Joanna and David each spend half of their time producing beef and half of their time producing corn