69. The process of urine formation involves the following steps: ①Active transport of sodium; water follows passively. ②Active transport of sodium; impermeable to water. ③Passively permeable to water only. ④ Passively permeable to water and urea. Please match each step to the following regions of the nephron: (A) 1a, 2c, 3b, 4e (B) 1a, 2b, 3c, 4 e (C) 1a, 2b, 3e, 4 c (D) 1b, 2a, 3e, 4 c