Answer:
D. 
Explanation:
The speed of sound in dry air is given approximately by

We can use the equation

to calculate the frequency.
Rearranging this gives us

.
So the frequency is

, which equals 0.42 hertz
(to 2 decimal places).
Answer:
c
Explanation:
The changing of color of a substance is not necessarily an indicator of a chemical change. For example, changing the color of a metal does not change its physical properties. However, in a chemical reaction, a color change is usually an indicator that a reaction is occurring.
Chemical change, we can't change the bread back the way it was before
Hope this helps!
Answer: True
Explanation:
A photo detector that can respond to the entire rang of visible light can be design, it is true.
Photo detector is a device in an optical receiver which receives optical signals and convert it to electric signal. It is the key device position in front of the optical receiver.