To find the density of an object, divide the mass by the volume. To find the density of the object in your problem, use a calculator to divide 60.0 by 400.0
I believe the correct answer would be C. Atmosphere
The smallest wave will have an amplitude of 3 if the two waves shown were to combine. Remember that amplitude is the maximum displacement of two waves and the result of two wave's amplitude is the sum of the two wave's displacements at the same location.