When is the relative velocity of two moving cars zero?

Which one of the following time-displacement graphs represents two moving objects $P$ and $Q$ with zero relative velocity?

$A$ train is moving at a constant speed $V$. When its driver observes another train in front of him on the same track and moving in the same direction with constant speed $u$. If the distance between the trains is $x$,what should be the minimum retardation of the train to avoid a collision?

$A$ person sitting in a moving train with his face towards the engine,throws a coin vertically upwards. The coin falls ahead of the person. The train:

An express train is moving with a velocity $v_1$. Its driver finds another train moving on the same track in the same direction with velocity $v_2$. To escape a collision,the driver applies a retardation $a$ to the train. The minimum time to escape a collision will be:

Ball $A$ is thrown vertically upward with a speed of $10 \, m/s$. At the same instant,another ball $B$ is released from rest at a height $h$. At time $t$,the speed of $A$ relative to $B$ is ...........