Usually in the deep sea and underwater caves where there is no light
Answer:

Explanation:
Givens



The frequency is defined by

Where
is the speed of the wave in the string and
is its wave length.
The wave length is defined as 
Now, to find the speed, we need the tension of the wire and its linear mass density

Where
and the tension is defined as 
Replacing this value, the speed is

Then, we replace the speed and the wave length in the first equation

Therefore, the frequency is 
Answer:
True
Explanation:
Magnetic field lines outside of a permanent magnet always run from the north magnetic pole to the south magnetic pole. Therefore, the magnetic field lines of the earth run from the southern geographic hemisphere towards the northern geographic hemisphere.
Wave speed = frequency * wavelength
Wave speed = 4 * 25
Wave speed = 100 m/s