Answer:
A. voltage
Step-by-step explanation:
The rate of heat transfer per unit area in a thermal circuit is equivalent to the voltage in an electrical circuit.
This is because, within an electrical circuit the voltage is supplied by a generator, which can be batteries, batteries, or others. The moment the electrical voltage acts within a circuit, it will consist of the gradient of electrical potential between two points, which will cause the electrons to move from the point with the highest concentration to the point with the lowest concentration (similar to the transfer of heat from the thermal circuit) generating the electric current.