Which pair correctly matches RC and RL time constants?

• A. RC: t = RC; RL: t = L/R
• B. RC: t = R/C; RL: t = R/L
• C. RC: t = RC^2; RL: t = L^2/R
• D. RC: t = CR; RL: t = RL

Answer: (A)

Time constants describe how quickly a first‑order circuit responds. For an RC circuit, the characteristic time constant is the product R·C, which sets the rate at which the capacitor voltage changes toward its final value. For an RL circuit, the characteristic time constant is L/R, which sets the rate at which the inductor current builds up or decays. So the correct pairing is RC with t = RC and RL with t = L/R. Other options mix up the expressions or use incorrect units (such as R/C, RC^2, or RL), which don’t represent the standard time constants.