The increase in osmolarity from outer to inner medullary interstitium is maintained due to:
$(i)$ Close proximity between Henle's loop and vasa recta
$(ii)$ Counter current mechanism
$(iii)$ Selective secretion of $HCO_{3}^{-}$ and hydrogen ions in $PCT$
$(iv)$ Higher blood pressure in glomerular capillaries

• A
  $(i)$ and $(ii)$
• B
  Only $(ii)$
• C
  $(iii)$ and $(iv)$
• D
  $(i), (ii)$ and $(iii)$