If one of two interacting charges is doubled, the force between the charges will double.
Explanation:
The force between two charges is given by Coulomb's law

K=constant= 9 x 10⁹ N m²/C²
q1= charge on first particle
q2= charge on second particle
r= distance between the two charges
Now if the first charge is doubled,
we get 
F'= 2 F
Thus the force gets doubled.
Yes , increased tension suggests increased molecular attraction between the molecules of the ropes which affect the increase in the speed of wave.
It’s 4 because a coiled springs is closely spaced then widen
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