The change of color. Each color has a different wavelength therefore a different frequency.
Answer:C
Explanation:
When a constant horizontal force is applied to the box, box started moving in the horizontal direction such that it moves with constant velocity 
Constant velocity implies that net force on the box is zero
i.e. there must be an opposing force which is equal to the applied force and friction force can serve that purpose.
So option c is the correct choice.
Third
every action has an opposite and equal reaction
Answer:
The answer to the question is 7200