Which of the following is not consistent with facilitated diffusion?
rapid movement of polar molecules across a membrane.
solute movement can only proceed down a concentration gradient.
two different solutes can move through the same transporter, in the same or opposite directions.
there can be solute movement across the membrane that does not change vm.
solute movement proceeds against a concentration gradient as long as there is a specific transporter.

Solute movement can only proceed down a concentration gradient.

Explanation:

In cell membrane, solute can be transported by various means from passive to active transport systems.

During facilitated diffusion however, solutes are HELPED to be transported AGAINST concentration gradient (from a region of low to high concentration) across the cell membrane by a special kind of vehicle (PROTEIN) present within the cell membrane.

Thus, it is NOT correct to say Facilitated diffusion occurs ONLY down the concentration gradient

the answer is d.) homologous; have mammary glands