As shown in the diagram, an electric circuit consisting of an ammeter, a voltmeter, 4 cells of 1.5 V each, a…

As shown in the diagram, an electric circuit consisting of an ammeter, a voltmeter, 4 cells of $1.5$ V each, a plug key with a gap XY was set up. Voltmeter and ammeter readings were recorded in the observation table for four arrangements as given below: Arrangement No. 1 - only resistor $R_1$ in gap XY Arrangement No. 2 - only resistor $R_2$ in gap XY Arrangement No. 3 - Resistors $R_1$ and $R_2$ in series in gap XY Arrangement No. 4 - Resistors $R_1$ and $R_2$ in parallel in gap XY Based on the observations, four $V$-$I$ graphs A, B, C and D as shown in figure were drawn. Study these graphs.