The equation of the sphere concentric with the sphere x² + y² + z² −4x − 2y − 6z − 7 = 0 and passing through (0,0,0) is.

a. x² + y² + z² − 4x − 2y − 6z = 0
b. x² + y² + z² − 4x − 2y = 0
c. None of these
d. x² + y² + z² = 0