This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Diffusion of gas in the respiratory membrane 
Select and explain. 
A. Directly proportional to thickness of respiratory membrane, B. Inversely proportional to molecular weight of the gas, C. No lipid - water solubility, D. Inversely proportional to pressure gradient
B: Inversely proportional to molecular weight of the gas
Answer: B: Inversely proportional to molecular weight of the gas 
Explanation: The rate of diffusion of gas across the respiratory membrane is
-        Directly proportional to 

The pressure gradient across the membrane
Lipid solubility of the gas 
The surface area of the membrane

-        Inversely proportional to

The thickness of the membrane​
The molecular weight of the gas