13. (LO 6) Assume that an economy can produce either 300 tonnes of coffee and no rubber, or 100 tonnes of rubber and no coffee. You may further assume that this 3:1 coffee/rubber ratio is constant. Draw a production possibilities curve for this economy with coffee on the vertical axis. Next, indicate with the letters a and b an increase in rubber production. Finally, illustrate with a triangle the cost of this additional rubber.