Answer:
= 391.67 Hz
Explanation:
The sound of lowest frequency which is produced by a vibrating sting is called its fundamental frequency (
).
The For a vibrating string, the fundamental frequency (
) can be determined by:
= 
Where v is the speed of waves of the string, and L is the length of the string.
L = 42.0 cm = 0.42 m
v = 329 m/s
= 
= 
= 391.6667 Hz
The fundamental frequency of the string is 391.67 Hz.
Since the two taped poles of the magnets labeled A and B attracts one to each other, we know that the two taped poles are oppsosite.
So, you can predict with total certainty that when she brings the taped end of the third magnet (magnet C) near each of the first two magntes, in one case they will attract each other and in the other case they will repele mutually.
You are certain of that because, since the taped poles of the first two magnets are opposite, the pole of the third magnet has to be equal to one of the two first taped poles and opposite to the other of the two firest taped poles.
The answer would be C) because weight lifting is used to increase your muscular strength, and that's why you try and add more weight during sessions, so it's not too easy for you and you can build a higher endurance.