Building Assemblies Are Generally More Vapor Permeable Than Air Permeable

Multiple Choice

Building assemblies are generally more vapor permeable than air permeable because

A) vapor is not really a gas.
B) vapor is only a small part of the air- vapor mixture.
C) vapor has only water molecules, while air is a mixture of nitrogen and several other gases.
D) vapor pressure differential between the inside and the outside air is much greater than the corresponding air pressure differential.
E) vapor molecules are heavier than air molecules.