The foci and the directrices of the hyperbola are labeled. Which equation represents the hyperbola?
a. x^2 - y^2 = 9
b. (x + 3)^2 - y^2 = 1
c. (x - 3)^2 - y^2 = 1
d. x^2 + (y - 3)^2 = 9