For a zero order reaction, instantaneous rate is always equal to the average rate. Why?

Instantaneous rate of a reaction is determined by drawing a tangent at time t on either of the curves for concentration of reactant and product vs time t and calculating its slope. For a zero order reaction, the plot of concentration of reactant vs time is shown as follows

As can be seen from the above figure, the plot of concentration of reactants vs time gives a straight line graph. hence, it is not possible to determine the draw a tangent and determine the instantaneous rate. We can only determine the average rate.