Which two layers of earth are mainly composed of metallic elements such as nickel and iron, which are abundant in meteoroids and comets? inner core outer core lower mantle upper mantle crust

The correct answer is the inner core and outer core.  

Similarly to other terrestrial planets like Venus, Mercury, and Mars, the Earth comprises many layers. This is the outcome of it going through planetary differentiation, where the denser substances sink to the middle to form the core, while the lighter substances get produced around the outside. The core, that is, the outer core and the inner core comprises mainly of nickel and iron, while the upper layers of the Earth comprise minerals and silicate rock.  

explanation:

most inclusive taxonomic category, larger than a kingdom. largest taxonomic group, consisting of closely related phyla. group of closely related species, first part of the scientific name in binomial nomenclature. group of similar organisms that breed and produce fertile offspring.

both anaerobic and aerobic respiration use glucose (or other carbohydrates), proteins and lipids to produce atp and nadh. cell uses those energy molecules, obtained from food and   breaks them down during the chemical reactions of respiration. in anaerobic respiration, nadh is regenerated to nad+ by enzymes lactate dehydrogenase (lactate is produced, e.g. in muscle cells) or by alcohol dehydrogenase where ethanol is produced (alcohol fermentation), while in aerobic respiration, electrons from nadh are deposited in the mitochondria to regenerate nad+ (electron transport chain. anaerobic respiration produces 2 atp per molecule of glucose compared to aerobic respiration where about 38 atp molecules are produced.