Ingvar and Olaf are the only two fishermen in their area. Each has been assigned an ITQ that allows him to catch 20 tons of salmon. Ingvar's $\mathrm{MC}$ of catching salmon is $\$ 6$ per ton while Olafs MC of catching salmon is $\$ 7$ per ton. If the price of salmon is $\$ 10$ per ton, then to maximize efficiency, the two guys should trade ITQs until Ingvar is in charge of catching _____ tons while Olaf catches ____ tons.
a. $20 ; 20$
b. $30 ; 10$
c. $40 ; 0$
d. $0 ; 40$