Douglas exercised a nonstatutory stock option on April 30, 2018. He purchased 1,000 shares of his company's stock for $10,000. The fair market value of the stock at the time of purchase was $15,000. He included the $5,000 difference between the fair market value and the purchase price as compensation income when he filed his 2018 return. What is Douglas's basis in the underlying stock?

A: $0

B: $5,000

C: $10,000

D: $15,000