Is there multiple answer choices?
Beats can be defined as the periodic fluctuations in the frequency of sound waves. That is option D
<h3>What are sound waves?</h3>
Sound waves are those waves that are produced by the vibration of an object whose energy is usually propagated through a medium.
When two sound waves of different frequencies meets, a periodic variation that occurs is called beats.
Therefore, Beats can be defined as the periodic fluctuations in the frequency of sound waves.
Learn more about waves here:
brainly.com/question/15663649
#SPJ1
Answer:

Explanation:
given,
de broglie wavelength of the particle = 2.1 × 10⁻¹⁰m
Kinetic energy increase factor = 3
de broglie wavelength of new particle = ?
we know,

Kinetic energy

...........(1)
Kinetic energy of other particle
............(2)
dividing equation (1)/(2)




Opposites attract while like charges repel. :)