Why do motors have gaps or breaks throughout the rotor and commutator?

a) It makes the motor cheaper since not as much material is needed.
b) Only some sections of the rotor will act as electromagnets, causing the rotor too always be pushed in the same direction.
c) It allows some of the magnetic field to escape so that rotor doesn't get too stressed.
d) It lets air into the motor to keep it cool since the electricity can make it hot.
e) It happens when rotors don't use enough oil, so they crack.