What Best Explains the Double-Headed Arrows Associated with Carbon Dioxide

What best explains the double-headed arrows associated with carbon dioxide and molecular oxygen shown in the following illustration?

A) Individual molecules are unable to cross the membrane,but they can be found in relatively large numbers on both sides.
B) The membrane is permeable to these small molecules and they enter or leave rapidly as their concentration gradients change.
C) An isotonic condition exists for each molecule,so no net movement takes place.
D) Homeostasis requires that surplus molecules entering the cell be rejected and returned to the outside.