Answer:
I think the answer must be A.
Answer: A) Transformer
Explanation: A Transformer is an apparatus, device or a component in a system that is used to convert high voltage into low voltage. It is used to either increase or decrease the voltage of an alternating current.
A transformer uses the basic principle of electro magnetic induction, having two or more coils, the voltage is changed from one coil to another but with thesame frequency as alternating current energy passes through them.
Answer:

Explanation:
<h3><u>Given data:</u></h3>
Frequency = f = 200 Hz
Velocity = v = 400 m/s
<h3><u>Required:</u></h3>
Wavelength = λ = ?
<h3><u>Formula:</u></h3>
v = fλ
<h3><u>Solution:</u></h3>
Put the givens in the formula
400 = (200)λ
Divide 200 to both sides
400/200 = λ
2 m = λ
λ = 2 m
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Explanation:
The two postulates of special theory of relativity
Postulate 1: The law of physics are invariant under any of inertial frame of reference.
Postulate 2: The velocity of light is remains same in each ans every frame of reference and independent of relativity.
They are differ from classical mechanics that in classical mechanics there is no change in mass and length in relative velocity but in relativistic mechanics it changes.
These two postulates implements in phenomenon like time dilation , length contraction etc.
Thanks
Let the unknown distance be xmiles
x/39-x/72=11hr
72x-39x/2808=11hr
33x/2808=11
33x= 30888
x=936miles
U can substitue back to check
at speed of 72mph, he would need 936/72=13hrs
at speed of 39mph, he would need 936/39=24hr
the difference is 24-13=11