50 points answer correctly what are beats? a )periodic fluctuations in the velocity of sound waves b ) periodic fluctuations in the wavelength of sound waves c ) periodic fluctuations in the intensity of sound waves d ) periodic fluctuations in the frequency of sound waves

May someone explain how to do this question?

If a substance is in the gas phase, which of qualities of the gas will stay constant? a: volume b: mass c: shape d: position of particles

An electromagnetic wave with frequency 65.0 hz travels in aninsulating magnetic material that has dielectric constant 3.64 andrelative permeability 5.18 at this frequency. the electric fieldhas amplitude 7.20 \times 10^{ - 3} {\rm v}/{\rm m}. a. what is the speed of propagation of the wave? b.what is the wavelength of the wave? c.what is the amplitude of the magnetic field? d.what is the intensity of the wave?