In a circuit, a measured voltage drop across a switch (with the switch intended to be closed) most directly indicates that the switch is behaving as what kind of circuit element?

• A. An ideal short circuit with $R=0$, since a voltage drop proves perfect conduction
• B. An open circuit with $I=0$, since any measured voltage drop means no current can flow anywhere in the circuit
• C. A voltage source, because any device with a voltage across it must be supplying that voltage
• D. A resistor with nonzero resistance, since $V=IR$ implies a voltage drop when current flows

Answer: (D)