The Table Gives Marginal Product Data for Resources a and B

The table gives marginal product data for resources a and b. The output of these independent resources sells in a purely competitive market at $1 per unit. Assuming the prices of resources a and b are $5 and $8 respectively, what is the least costly combination of resources for the firm to employ in producing 192 units of output?

A) 2 of a and 6 of b
B) 6 of a and 2 of b
C) 4 of a and 3 of b
D) 3 of a and 4 of b