Please choose the correct answer to the question: 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?

Answer1: a pH imbalance Answer2: a pressure difference Answer3: a temperature increase Answer4: a concentration gradient

Answer format: answer1/answer2/answer3/answer4
Answer: answer4