Answer:
A. 22 Hz and 42 Hz
Explanation:
When two waves of slightly different frequency meet each other, they produce an interference pattern called beat. The frequency of this beat is equal to the absolute value of the difference between the individual frequencies of the two waves:
[tex]f_B=|f_1-f_2|[/tex]
In this problem, we have 20 beats per second, so the beat frequency is
[tex]f_B = \frac{20}{1 s}=20 Hz[/tex]
Therefore, we have to find the pair of waves whose frequency difference is 20 Hz. Let's analyze each choice:
A. [tex]f_B = |22 Hz - 42 Hz|=20 Hz[/tex] --> YES
B. [tex]f_B = |47 Hz - 55 Hz|=8 Hz[/tex] --> NO
C. [tex]f_B = |38 Hz - 63 Hz|=25 Hz[/tex] --> NO
D. [tex]f_B = |18 Hz - 26 Hz|=8 Hz[/tex] --> NO
So, the correct choice is A.