In the circuit shown in figure, XY is a potentiometer wire 100 cm long. The circuit is connected as shown. With switches S2 and S3 open, a balance point is found at Z. After switch ,S1 has remained closed for some time, it is found that contact at Z must be moved towards Y to maintain a balance. Which of the following is most likely reason for this?

(a) If V1 decreases, then potential difference across XZ will decrease. So to have same potential difference i.e., V2 across XZ, length XZ has to be increased or Z should be moved towards Y.(b) is incorrect because for this z should have been moved towards r.(c) will be incorrect because xz can get warm if current through it increases which is not possible.(d) is incorrect because there is no current through R1.