The Existence of a Hydrostatic Pressure Gradient in Phloem Tubes

Multiple Choice

The existence of a hydrostatic pressure gradient in phloem tubes can be accounted for by

A) the fact that the leaves are higher than the roots, which means that gravity creates pressure.
B) the diffusion of water from one sieve-tube cell to the next.
C) the loading of sugars into phloem at sources and removal of sugars at sinks.
D) the active transport of water from sugar sources to sugar sinks.