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.
An element refers to a collection of atoms having the same number of protons and electrons (an atomic number). In each element there is a different atomic number due to a different amount of protons in the nucleus.
An isotope is a variation of an element that contains a different number of neutrons, therefore adding weight to the atom.
An ion is a charged atom, and its charge shows how many electrons it needs to gain or lose in order to become stable.
There is no scientific way to prove that it happened it’s like a hypothesis without being able to test the hypothesis
The component that’s dissolved is called the solvent
This is known as destructive interference. In fact, if the two waves (with the same amplitude) are shifted by exactly half a wavelength when they merge together, then the crest of one wave will match up perfectly with the trough of the other wave, and they will cancel each other out.
Destructive interference occurs when the maxima of two waves are 180 degrees out of phase: a positive displacement of one wave is cancelled exactly by a negative displacement of the other wave. The amplitude of the resulting wave is zero. ... The dark regions occur whenever the waves destructively interfere.
