Use the midpoint formula in exercise 23 to find the center of a sphere if one of its diameters has endpoints A and B. Then find an equation of the sphere.

a) Center: Midpoint of AB; Equation: ((x - h)² + (y - k)² + (z - l)² = r²)
b) Center: Endpoint A; Equation: ((x - h)² + (y - k)² + (z - l)² = r²)
c) Center: Midpoint of AB; Equation: (x² + y² + z² = r²)
d) Center: Endpoint B; Equation: ((x - h)² + (y - k)² + (z - l)² = r²)