A ring of radius r and mass m rotates about an axis passing through its center and perpendicular to its plane with angular velocity ω. Its kinetic energy is:
a) 1/2 * m * ω² * r²
b) 1/4 * m * ω² * r²
c) 1/6 * m * ω² * r²
d) 1/8 * m * ω² * r²