Task: Question Answering
Input (English): In the human kidney, urea from the blood is filtered through the glomerular membrane into a nephron. The movement of urea across this membrane occurs without an input of energy. Which factor is the most likely reason urea absorption does not require energy?  
 (A) a pH imbalance (B) a pressure difference (C) a temperature increase (D) a concentration gradient
Output (English): D