Which of the following distance-time graphs shows a truck moving with speed which is not constant ?

$(i)$ In a $distance-time$ graph, the constant speed of a truck will be represented by a straight line.

$(ii)$ In a $distance-time$ graph, a straight parallel to the time axis indicates that the truck is not moving.

$(iii)$ A curved line on the $distance-time$ graph indicates that the truck is moving with a speed which is not constant.

$(iv)$ In a $distance-time$ graph, the constant speed of a truck will be represented by a straight line.

Hence, option $c$ is correct answer.