I believe it is either B or A, personally i'm leaning towards B, but at the same time, i don't think that we would be obtaining materials via pollination, so it then causes A to make more sense.
A gateway substance is something that may be menial but could lead to something bigger, an example is weed in the drugs world, weed is not hard to get and is usually a gateway drug to stronger types of drugs and potentially more dangerous
Answer;
-Simple squamous epithelium
Explanation;
-Simple squamous epithelium is the single layer of thin, flat cells that line body surfaces which are not exposed substantially abrasive forces. The primary function of the columnar cell is that secretion and absorption simple squamous epithelium a single layer flat scale shaped cells.
-Simple squamous epithelium lines the air sacs, or alveoli, of the lungs. The alveoli are sites where air is exchanged in the lungs. Simple squamous epithelial cells in the alveoli allow oxygen from the air to enter the blood in the capillaries of the lung.
Answer:
Human activities have a tremendous impact on the carbon cycle. Burning fossil fuels, changing land use, and using limestone to make concrete all transfer significant quantities of carbon into the atmosphere
