Profit = income-expenditure, and then calculate the profit of A B C separately.

A:1200 *1000/400 * (3.5-0.8) =1

b: 1200 * 1000/300 *(2.8-0.6)= 1

c: 1200 * 1000/200 *( 1.9-0.5)= 1

Analysis shows that B has the largest profit.