Which of the following correctly identifies the most productive layer of a soil profile? topsoll, just below undecomposed organic matter
subsoll, between topsoil and parent material
undecomposed organic matter, at the very top
parent material, just above bedrock

Just below the O horizon is “topsoil” or “A” horizon.
Usually it is darker than the lower layers, loose and crumbly with varying amounts or organic matter. Plants roots, bacteria, fungi, and small animals are abundant here, and plants thrive in it. It is generally the most productive layer of soil.

Air enters through the nose (and sometimes the mouth), moves through the nasal cavity, the  pharynx, the  larynx, enters the  trachea, moves through the  bronchi  andbronchioles  till the alveoli.