Which of the following is the equation for a circle with center (5, -1) and radius of 4?
a. (x + 5)2 + (y - 1)2 = 16
b. (x + 5)2 + (y - 1)2 = 4
c. (x - 5)2 = (y + 1)2 = 4
d. (x - 5)2 + (y + 1)2 = 16