Which Features Are Required for Efficient External Respiration to Take

Which features are required for efficient external respiration to take place?

A) The gas exchange region must be moist,thin and have a large surface area.
B) The gas exchange region must be dry,thin and have a large surface area.
C) The gases being exchanged must be present in a low concentration in order for diffusion to occur.
D) Gas exchange can only occur at a temperature lower than the body temperature of the organism.
E) The gas exchange needs to occur in an aquatic environment in order to be efficient.