[单选题]
A generator coil rotatest 7s27b8s h-pwey e,ob-/k7 xinhv . dbvhm0ct0u, xj1 through 60 revolutions each second. The frequency of the0j /i xv7xh.0cn-v ,dkmb1thu $emf$ is
A. 30 Hz.
B. 60 Hz.
C. 120 Hz.
D. cannot be determined from the information given.
