Which two factors affect how many gas molecules move across the respiratory surface in any given ...

Which two factors affect how many gas molecules move across the respiratory surface in any given
  time?
  a. moisture and transport proteins
  b. moisture and surface area
  c. partial pressure gradient and moisture
  d. partial pressure gradient and surface area
  e. transport proteins and surface area

Question 2

How does hemoglobin help to maintain the pressure gradient?
  a. by duplicating oxygen
  b. by carrying oxygen away from the lungs
  c. by feeding oxygen into the lungs
  d. by mixing oxygen with carbon dioxide
  e. by binding with nitrogen