6. The circuit in the figure consists of three resistors and three emf sources, with
R = 1 k?, ?? = 10 V, and E? = 5V (note the direction of this emf). The power
dissipated in the resistor in the middle, the one whose resistance is R, is found
to be zero. What is the value of E??
(1) 5 V
(2) 10 V
(3) 0
(4) -2.5 V
(5) 7.5 V