A circle in the standard (x, y) coordinate plane is tangent to the x-axis at 4 and tangent to the y-axis at 4. Which of the following is an equation of the circle?

a
(x − 4)2 + (y − 4)2 = 16

b
x2 + y2 = 16

c
(x + 4)2 + (y + 4)2 = 16

d
x2 + y2 = 4

e
(x − 4)2 + (y − 4)2 = 4