The answer is two (4 -3)
Zero is not a SF
Answer:

Explanation:
As we know that the speed of the sound is given as

now at t = 273 k = 0 degree

so we have


now when temperature is changed to 313 K we have

now we have



now from two equations we have

so we have


To answer that question, we don't care what the highest and lowest
levels of the wave are, or how far apart they are. We only need to be
able to identify the highest point on the wave, and keep track of how
often those pass by us.
You said it takes 4 seconds for a complete wave to pass by.
Through the sheer power of intellect, I'm able to take that information
and calculate that 1/4 of the wave passes by in 1 second.
There's your frequency . . . 1/4 per second, or 0.25 Hz.