A coil, rotating in a uniform magnetic field, has an induced emf given by the equation ε=20cos60πt . If the same coil is now rotated in the same magnetic field at twice the original angular frequency, which one of the following equations correctly gives the induced emf in the coil?
a. ε=20cos60πt
b. ε=20cos120πt
c. ε=40cos60πt
d. ε=40cos120πt