A clock pendulum is timed as it swings back and forth. The clock is started when the bob is at the left end of its swing, when the bob returns to the left end for the 90th return the clock reads 60 s; What is the frequency of vibration? O a. 0.667 Hz O b. 60 Hz O c. 0.01667 Hz Od. 1.5 Hz O e. None Clear my choice T=0.0667