When a Person Undergoes Renal Dialysis, a Machine Is Used

When a person undergoes renal dialysis, a machine is used to replace the function of the kidneys.Substances to be removed from the blood move across the dialysis membrane into the dialysis solution.To be maximally effective at removing urea from the blood, the concentration of urea in the dialysis solution should be

A) the same as it is in the blood.
B) slightly lower than it is in the blood.
C) slightly higher than it is in the blood.
D) zero.
E) much higher than it is in the blood.