`A` and `B` are two objects moving with velocities `overlineV_A` and `overlineV_B`. What is the velocity of `A` relative to `B ?`

The relative velocity of a body A with respect to a body B is the time rate at which body A changes its position with respect to body B . a)If V_A and V_B are the velocities of A and B moving in opposite directions,what is the relative velocity of A with respect to B ? b)Two trains along the same straight rails are moving with constant velocity of 60km/h and 30km/h towards each other.If at time t=0 ,the distance between them is 90km ,find the time when they collide. c)The velocity time graph of two bodies A and B make angles of 30° and 60° with the time axis,what is the ratio of their acceleration?

A catalyst is used : only for increasing the velocity of the reaction, for altering the velocity of the reaction, only for decreasing the velocity of the reaction, all (a), (b) and (c ) are correct