Answer:
I would say: D.
Explanation:
["Frequency is the number of times an object vibrates in 1 second; frequency is also measured in hertz and is recognized by your ears as differences in pitch."] Frequency is how many times an object vibrates in ONE second. So that knocks the answer down to either A or B. Frequency itself is also measured in hertz, which knocks the answer to: D. hertz per second.
