It is true because <span>A pyramid of biomass is a representation of the amount of energy contained in biomass, at different trophic levels for a given point in time . The amount of energy available to one trophic level is limited by the amount stored by the level below. Because energy is lost in the transfer from one level to the next, there is successively less total energy as you move up trophic levels. Tree is a base as it provides food and energy.</span>
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.
<span>Suppose that an electric charge is produced on one part of a body. If the charge spreads through the entire body, the body is most likely made of metal. </span>
No the substance will remain the same substance as before.
Friction is the force you get when you (for example) Rub something with another, it's a force that may generate heat and even some resistance. Another example is rubbing your hands together, they get hot, therefore friction is working, without friction you wouldn't be able to stop moving.
