21. Two identical electrical point charges `Q`, separated by a distance `d` produce an electrical force of `F` on one another. If the distance is decreased to a distance of `0.40d`, what is the strength of the resulting force?
(A) `6.3F`
(B) `2.5F`
(C) 0.40 F
(D) `0.16F`
(A) `6.3F`
(B) `2.5F`
(C) 0.40 F
(D) `0.16F`