It would be, 1.000. Hope that helps :)
Scientists have designed solar cells to trap solar energy and convert it to D electrical energy. Its D because converting something has to do with recharging something and electronics have to charge. I hope I helped.
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.