A cell has a membrane potential of –100 mV (more negative inside than outside) and has 1,000 times more calcium ions outside the cell than inside. Which of the following best describes a mechanism by which Ca²⁺ enters the cell?

• A. Movement of Ca²⁺ into the cell through an ion channel down its concentration gradient
• B. Passive diffusion of Ca²⁺ into the cell down its electrochemical gradient
• C. Cotransport of Ca²⁺ into the cell with Cl^–
• D. Movement of Ca²⁺ into the cell through a carrier protein down its electrical gradient
• E. Facilitated diffusion of Ca²⁺ into the cell down its electrochemical gradient