From the diagram we may note that movement along the demand curve is caused by change in price of a commodity. When price of the commodity increases from Sh.10 to Sh.20, quantity demanded reduces from 20 to 15.This leads to movement along the demand curve from point C to B representing a decrease in quantity demanded.

On the other hand, when price of the commodity reduce from Sh.10 to Sh.5,quantity demanded increase from 20 to 30 leading to movement along the demand curve from point C to A representing an increase in quantity demanded.