The cost function c(w1, w2, y) of a firm gives the cost of producing y units of output when the wage of factor 1 is w1 and the wage of factor 2 is w2. Find the cost functions for the following firms:

a. A firm with production function f(x1, x2) = min{2×1, 3×2}

b. A firm with production function f(x1, x2) = 2×1+ 3×2

c. A firm with production function f(x1, x2) = max{2×1, 3×2}