A: If two particle are neither approaching towards nor receding away from other then their relative velocity is zero.
R: Relative velocity of 1 w.r.t. 2 is given by `vec(v)_(12) = vec(v)_(1) - vec(v)_(2)`.
A. If both A and B are true and R is the correct explanation of A
B. If both A and R are true but R is not correct explantion of A
C. If A is true but R is false
D. If A is false but R is true