A circuit has a cell, a key and two resistors R1 and R2 connected in series in that order. The ends of R2 are labelled X (nearer to R1) and Y (nearer to the key). To measure the potential difference across R2, between which two points should a student connect the terminals of a voltmeter?
a) Cell positive terminal and point X
b) Point X and point Y
c) Cell negative terminal and point Y
d) Across the key and cell positive terminal