A car travels through a valley at constant speed, though not at constant velocity. Explain how this is possible.

Explanation:

An object following a circular path can be covering the same distance along the circle's circumference with every passing minute, giving it a constant speed. Since a change in either speed OR direction means a change in velocity, the object's velocity is not constant.

velocity is a vector so therefore direction affects it being constant.

An object's acceleration is the rate its velocity (speed and direction) changes. Therefore, an object can accelerate even if its speed is constant - if its direction changes. If an object's velocity is constant, however, its acceleration will be zero.  

because growing process had stoped after 4-5 year

it is a negative acceleration