Kingdom because there are 3 kingdoms - Eukarea, Prokarya and Archaea
The answer is letter a. loop
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The bottom of the sea is lined with calcium carbonate, which comes from the shells of zooplankton that have died and sunk to the ocean floor. ... As a result, according to a study published recently in PNAS, parts of the seafloor are disintegrating.
The answer you are looking for is C. Nitrogen. Commonly found as Nitrite
(N3) or Nitrate (N2) nitrogen is a key element required by all life. Nitrogen is most often released back into the atmosphere by micro-organisms during the decomposition of organic matter, and into the ground through decomposition and defecation (Pooping).