Answer
3.0 V.
Zero volts.
Work Step by Step
Suppose battery A’s negative terminal is placed atop battery B’s positive terminal. The point where they touch (point C) is 1.5 volts higher than battery B’s negative terminal. Moreover, battery A’s positive terminal is 1.5 volts higher than its own negative terminal at point C, which places it 3.0 volts higher than battery B’s negative terminal.
Suppose battery A’s positive terminal is touching battery B’s positive terminal. The point where they touch (point C) is 1.5 volts higher than battery B’s negative terminal. However, the point where they touch is also 1.5 volts higher than battery A’s negative terminal. Hence there is no potential difference at all between the exposed ends, which are the negative terminals.
