Why does countercurrent exchange optimize gas exchange across some gills?

Why does countercurrent exchange optimize gas exchange across some gills?
 
  a. It sustains an optimal thermal environment for gas exchange.
  b. It optimizes perfusion to the respiratory surface.
  c. It sustains an optimal respiratory gas concentration gradient, allowing diffusion to occur
  across the entire respiratory surface.
  d. It optimizes delivery of the respiratory medium to the respiratory surface.
  e. It minimizes diffusional area.

Question 2

Which respiratory surface lacks physical protection from the external environment?
 
  a. tracheal system tubes
  b. human lungs
  c. external gills
  d. internal gills
  e. bird lungs