$(i)$ Velocity$-$time graph is a straight line inclined to the time axis from $0$ to $A$. This represents a uniformly accelerated motion.

$(ii)$ Velocity$-$time graph is a straight line parallel to time axis from $A$ to $B$. This represents uniform motion.

$(iii)$ The velocity$-$time graph is a straight line from $B$ to $C$ having a negative slope. This represents a uniformly retarded motion.

$(iv)$ Change in velocity $=40-0=40 m s ^{-1}$

Change in time $=10-0=10 s$

$a=\frac{\text { Change in velocity }}{\text { Change in time }}$

$=\frac{40}{10}=4 m s ^{-2}$

$(v)$ Since motion is uniform $a=0$

$(vi)$ Change in velocity $=0-40$

$=-40 m s ^{-1}$

Change in time $=50-30=20 s$