A body is thrown vertically upwards. Which one of the following graphs correctly represents the velocity vs time?

A rocket is moving in a gravity-free space with a constant acceleration of $2{ \mathrm{m} \mathrm{s}}^{-2}$ along $+x$ direction (see figure). The length of a chamber inside the rocket is $4 \mathrm{m}$. A ball is thrown from the left end of the chamber in $+x$ direction with a speed of $0.3{ \mathrm{m} \mathrm{s}}^{-1}$ relative to the rocket. At the same time, another ball is thrown in $-x$ direction with a speed of $0.2{ \mathrm{m} \mathrm{s}}^{-1}$ from its right end relative to the rocket. The time in seconds when the two balls hit each other is