20) A wave is a vibration in
A) space.
B) time.
C) both
D) neither

21) A wave having a frequency of 1000 Hz vibrates at
A) less than 1000 cycles per second.
B) 1000 cycles per second.
C) more than 1000 cycles per second.

22) A weight suspended from spring bobs up and down one complete cycle every 2 s. Its frequency is
A) 0.5 Hz.
B) 1 Hz.
C) 2 Hz.
D) none of the above