To find R_th, what must you do to the independent sources?

• A. Open all independent sources
• B. Replace independent voltage sources with short circuits and independent current sources with open circuits
• C. Short all circuits
• D. Remove all sources from the circuit

Answer: (B)

When determining Thevenin resistance seen from two terminals, you must turn off all independent sources so they no longer affect the network. For an ideal independent voltage source, this means replacing it with a short circuit (0 V). For an ideal independent current source, you replace it with an open circuit (no current). After these substitutions, you calculate the equivalent resistance looking into the terminals. If dependent sources are present, they stay active and you may need to apply a test source to find the resistance.

This is why replacing independent voltage sources with shorts and independent current sources with opens is the correct procedure—it removes the influence of the sources while preserving the passive resistance around the terminals.