Because if you plug those numbers into the graph, only C and D fit. Now we have to look at the unit price. If LaMar buys 2 lbs and pays 3.00 dollars, we know that the unit price is 3/2=1.5 dollars per pound. You could also check with the other peoples. $7.5/5lbs=1.5 dollars. $4.5/3lbs=$1.5