In a single closed loop a student begins at point P with the potential defined as 0 V and records the following sequence of potential changes while moving clockwise:
• rise of 9 V across battery B1
• drop of 4 V across resistor R1
• drop of 2 V across resistor R2
• drop of unknown magnitude across battery B2 (traversed from its positive to negative terminal)
Upon returning to point P, the potential must again be 0 V. What is the magnitude of the potential change across battery B2?
a) 5V
b) 3V
c) 7V
d) 1V