In a lab experiment with isolated lung cells you observe a linear uptake in choline from the ...

In a lab experiment with isolated lung cells you observe a linear uptake in choline from the extracellular medium by the cells until the extracellular concentration of choline reaches 200 nM. At this concentration the curve levels off and the same rate of uptake is seen at 200 and 225 nM extracellular choline. What has happened that can explain this observation?
 a. Cellular and extracellular choline concentrations are at equilibrium.
  b. The transporters for choline saturate at 200 nM.
  c. At choline concentrations of 200 nM and above, the osmotic pressure of the cytoplasm prevents additional choline from entering the cell.
  d. The cell cannot synthesis enough ATP to keep up with the energy demand of the carrier.
  e. None of these.