Auxin triggers the acidification of cell walls, which results in ...

[xclash]

Auxin triggers the acidification of cell walls, which results in rapid growth, but also stimulates sustained, long-term cell elongation. What best explains how auxin brings about this dual growth response?
A) Auxin binds to different receptors in different cells.
B) Different concentrations of auxin have different effects.
C) Auxin causes second messengers to activate both proton pumps on the plasma membrane and certain genes within the same cells.
D) The dual effects are due to two different types of auxins that are produced by different genes.
E) Other antagonistic hormones modify auxin's effects.

[clippers!]

I chose C and got the answer, hope that helps.