Please Help!
Sound waves are traveling through the air. Every ½ second, the sound waves pass a detector. Which of the following choices correctly describes the period and frequency of these waves?

The period is 2 s, and the frequency is 2 Hz.
The frequency is ½ second, and the period is 2 Hz.
The period is ½ second and the frequency is ½ Hz.
The period is ½ second, and the frequency is 2 Hz.

Please Help!