An electric bulb has onboard instruments that report the total electricity consumed since the start of the trip as well as the total distance covered.During a single day of operation,the bus travels on stretches M,N,O,and P, in that order.The cumulative distance travelled and the corresponding electricity consumption are shown in the table below:

$$\begin{array}{|l|l|l|} \hline \text{Stretch} & \text{Cumulative distance (km)} & \text{Electricity used (kWh)} \\ \hline M & 20 & 12 \\ \hline N & 45 & 25 \\ \hline O & 75 & 45 \\ \hline P & 100 & 57 \\ \hline \end{array}$$

The stretches where the electricity consumption per $km$ is minimum is

1. $M$
2. $N$
3. $O$
4. $P$