Refer to the Graph Showing the Response of a Hydroponic

Refer to the graph showing the response of a hydroponic plant being transferred into a new solution.The dependent variable is the number of phosphate transporters per unit area of root epidermis as a function of time after transfer. Why does the plot line level off after the plant has spent 12 hours in the new solution?

A) The roots have absorbed all of the phosphate from the solution after 12 hours.
B) After 12 hours, the activity of the phosphate transporters reaches maximal levels and levels off.
C) The phosphate that is taken up by the transporters has all been assimilated after 12 hours.
D) The level of phosphate in the cells has decreased after 12 hours, shutting down transcription of the phosphate transporter genes.
E) The transcription rate of the phosphate transporter genes goes down after 12 hours, when the roots have adjusted to the phosphate level in the new solution.