In the Pressure-Flow Model, What Starts the Flow of Phloem

In the pressure-flow model, what starts the flow of phloem contents?

A) sugar is actively transported into sieve tubes and water follows by osmosis
B) water leaves the sieve tubes by osmosis, after which sugar is actively transported into the sieve tubes
C) water is actively transported into sieve tubes, and then sugar enters by osmosis
D) both water and sugar are actively transported into the sieve tubes
E) sugar is actively transported into companion cells and water follows by osmosis