**Core Concept**
The resting membrane potential (RMP) of smooth muscle cells is a critical factor in determining their contractility and relaxation. Smooth muscle cells have a unique RMP compared to other cell types due to their high membrane resistance and low input resistance.

**Why the Correct Answer is Right**
The RMP of smooth muscle cells is primarily determined by the balance between the inward movement of potassium ions (K+) and the outward movement of sodium ions (Na+). The resting membrane potential is typically around -40 to -60 mV in smooth muscle cells. This value is maintained by the presence of potassium channels, which allow K+ ions to leak out of the cell, and the sodium-potassium pump, which helps to maintain the ionic balance across the cell membrane.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a known ion channel or pump that contributes to the RMP of smooth muscle cells.
**Option B:** This option is incorrect because it refers to the action potential, which is a transient depolarization that occurs in response to a stimulus, rather than the resting membrane potential.
**Option C:** This option is incorrect because it refers to the RMP of nerve cells, which is typically around -70 mV due to the presence of a high concentration of potassium ions inside the cell.

**Clinical Pearl / High-Yield Fact**
Smooth muscle cells have a unique ability to adapt to changes in the extracellular environment, such as changes in ion concentration or pH, which allows them to regulate their contractility and relaxation in response to various physiological stimuli.

**Correct Answer: C. -70 mV**