A potentiometer circuit shown in the figure is set up to measure e.m.f. of a cell E. As the point P moves from X to Y the galvanometer G shows deflection always in one direction, but the deflection decreases continuously until Y is reached. In order to obtain balance point between X and Y it is necessary to

• A. Decreases the resistance R
• B. Increase the resistance R
• C. \[{{v}_{d}}=\text{constant}\] 
• D. Reverse the terminals of cell E