According to the passage, radon levels are tested in indoors because:
F. Radon levels are different in every area, but they are always the same indoors.
G. Radon accumulates in the air inside a home and poses a possible health threat.
H. Radon gas has a strong, unpleasant smell that can only be detected indoors.
J. Radon levels vary from season to season but are similar for most houses.