The Volume of Phloem Tissue in a Stem Is Typically

The volume of phloem tissue in a stem is typically much smaller than the volume of xylem tissue because:

A) the pressure gradient driving water flow in the phloem is metabolically generated and, therefore, can be much larger than the environmentally generated xylem gradient.
B) phloem conduits are much wider, and because flow is proportional to conduit radius raised to the fourth power, fewer phloem conduits are needed.
C) xylem dies quickly and, therefore, needs to be replaced frequently.
D) plants lose hundreds of H₂O molecules for each CO₂ gained.