If the distance between two point charges is tripled,the mutual force between them will be changed by what factor?

Question

Quizplus · Accepted Answer

According to Coulomb's law, the force between two point charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them. 
So, if the distance is tripled, the force will decrease by a factor of $(1/3)^2=1/9$. Therefore, the correct answer is D.

If the Distance Between Two Point Charges Is Tripled,the Mutual