The correct choice is
D. 22 Hz and 42 Hz.
In fact, the beat frequency is given by the difference between the frequencies of the two waves:
In this problem, the beat frequency is 
, therefore the only pair of frequencies that gives a difference equal to 20 Hz is
D. 22 Hz and 42 Hz.
Yes, If all the forces on it are balanced it will be in equilibrium regardless of whether it is in motion or not.
It's weird but technically correct to say that a radio wave can be considered a low-frequency light wave. Radio and light are both electromagnetic waves. The only difference is that radio waves have much much much longer wavelengths, and much much much lower frequencies, than light waves have. But they're both the same physical phenomenon.
However, a radio wave CAN'T also be considered to be a sound wave. These two things are as different as two waves can be.
-- Radio is an electromagnetic wave. Sound is a mechanical wave.
-- Radio waves travel more than 800 thousand times faster than sound waves do.
-- Radio waves are transverse waves. Sound waves are longitudinal waves.
-- Radio waves can travel through empty space. Sound waves need material stuff to travel through.
-- Radio waves can be detected by radio, TV, and microwave receivers. Sound waves can't.
-- Sound waves can be detected by our ears. Radio waves can't.
-- Sound waves can be generated by talking, or by hitting a frying pan with a spoon. Radio waves can't.
-- Radio waves can be generated by an alternating current flowing through an isolated wire. Sound waves can't.
Answer:
the energy from the sun travel to earth the answer is A .through the radiation
"Frequency" means how often something happens. When you say how many times it happens every SECOND, you can call the number Hertz, or Hz.
In the graph, there are 10 vibrations in 10 seconds, 20 of them in 20 seconds, and 30 in 30 seconds. This should pretty much shout at you "THERE'S ONE VIBRATION EVERY SECOND !". When you realize that, you can say the frequency of the vibration is 1 Hz.
The graph tells us nothing about the speed of the waves. I suspect there's another picture to the question that you didn't include. If the other part shows the wavelength, then we could figure out the speed. But with just what you're showing us, we can't.
