C.
Explanation: Because mirrors are lighter, and they are easier than lenses to make perfectly smooth.
PART A)
Here we know that
potential difference across the wire is

resistance of wire is

now by ohm's law



Part b)
Power rating is defined as rate of electrical energy
it is defined as

now we have


Answer:
|Δf| = 37.3 kHz
Explanation:
given,
peak velocity = 4 m/s
speed of the sound = 1500 m/s
frequency = 7 MHz




= 37.3 kHz
|Δf| = 37.3 kHz
hence, frequency shift between the opening and closing valve is 37.3 kHz
I attached the missing picture.
Let's analyze the situation as spring goes from stretched to unstretched state.
When you stretch the string you have to use force against ( you are doing work) this energy is then stored in the spring in the form of potential energy. When we release the spring the energy is being used to push the two carts. When the spring reaches its unstretched length its whole initial potential energy has been used on the carts, and this is the moment when two carts have maximum velocity.
The potential energy of compressed ( stretched) spring is:

The kinetic energy of two carts is:

So we have:

Momentum also has to be conserved:

Momentum before the release of the spring is zero so it has to stay zero. We plug this back into the expresion we got from law of conservation of energy and we get:

Now we go back to the momentum equation:
Remember that two of the same charge repel each other.
These two balls are repelling each other. So they must both have the <em>SAME charge</em>. (last choice on the list)