How does auxin binding to the auxin receptor trigger cell elongation?

A) The auxin receptor is a proton channel that opens in response to auxin binding, allowing protons to flow down their concentration gradient and into the cell.
B) A signal transduction cascade results in the opening of a water pore that allows water into the cell, causing it to swell.
C) A signal transduction cascade causes proton channels to open, and protons to flow down their concentration gradient and out of the cell.
D) The auxin receptor begins phosphorylating proton pumps, activating them to pump protons up their concentration gradient and out of the cell.
E) A signal transduction cascade causes additional proton pumps to be made, and those proteins pump protons up their concentration gradient and out of the cell.