[单选题]
A generator coil rotatemym 5j7y ,48jpd4kff js througi,a0+x9sn3nm;v tjlzhn 6lf 7h 60 revolutions each second. The frequency of t 6l ltnhzjnf,9x; ivm0s3+an 7he $emf$ is
A. 30 Hz.
B. 60 Hz.
C. 120 Hz.
D. cannot be determined from the information given.
