A high concentration of molecule X is present outside the cell, and it enters the cell by facilitated diffusion. The graph shows how the concentration of X inside the cell changes at $30\,\text{s}$ intervals. Why does the concentration of X inside the cell remain unchanged after $150\,\text{s}$?
- A1 and 2 only
- B1 and 3 only
- C2 and 3 only
- D3 only