Please Explain

A competitive firm uses two inputs and has a production function f (x1,X2) = 22x1 25 x2 25. The firm can buy as much of either factor as it likes at factor prices w1 = w2 = 1. The cost of producing y units of output for this firm is: (a) 2(y/ 22)2. (b) 22(x1 + x2) y. (c) (x1 + X2) / 22. (d) y / 44. (e) y2 / 44