You run a gypsum mine and make drywall. For every $1,000 worth of gypsum you have on hand, you incur $500 in up-front processing cost to make the gypsum into $1,800 worth of drywall. What is the return to turning gypsum into drywall?

a) cannot be determined without knowing the cost to mine the gympsum

b) 20%

c) 80%

d) 30%