Answer:
the answer is Fragrent compounds
Hi , these are the definitions
Chemistry; the science that deals with the composition and properties of substances and various elementary forms of matter.
aquatic system ; ecosystem in a body of water , communities and organisms that are dependent on each other.
depth ; the distance from the top to the bottom of something .
geography ; the study of the earth s physical features and the people ,plants , and animals.
light ; brightness from the sun or from a light.
medium ; between small and large in size.
period ; the name given to a horizontal row of the periodic table.
properties ; attributes of a substance.
refraction ; the change of direction of a ray of light , sound , heat ....
reflection ; reflection is when infrared waves bounce from a surface .
salinity ; a saline substance or a liquid that contains salt.
pressure ; measure of the force applied over an unit area .
photosynthesis ; the process used by plants , algae , and certain bacteria to harness energy from sunlight into chemical energy.
thermocline ; an abrupt temperature gradient in a body of water .
intertidal ; relating to the region between the hide tide mark and the low tide mark .
benthic ; relating to the bottom of a sea or a lake or to the organisms that live there .
pelagic ; relating to or living in the sea far from the shore.
epipelagic ; relating to or inhabiting the uppermost layer of the water column of the open ocean , into which enough sunlight enters for photosynthesis to take place.
mesopelagic ; relating to or inhabiting the layer of the water column in the open sea that lies between the epipelagic and bathypelagic layers at depths of about 200 to 1,000 meters.
bathypelagic ; relating to or inhabiting the layer of the water column of the open sea that lies between the mesopelagic and abyssopelagic layers at depths of about 1,000 to 4,000.
abyssopelagic ; refering to or occurring in the region of deep water above the floor of the ocean .
neritic ; region of water lying directly above the sub littoral zone of the sea bottom .
photic ; pertaining to the generation of light by organisms .
aphotic; zone of an ocean , lowest level at which photosynthesis can take place .
commensalism ; type of a relationship between a plant , an animal , fungus , etc.
competition ; the struggle among organisms .
freshwater systems ; subset of earth s aquatic ecosystems .
marine systems ; inside aquatic system , marine systems include oceans , seas ,etc.
mutual ism ; relation between species of organisms in which both benefit from the association.
organism ; a system with many parts that depends on each other and work together.
parasitism ; the behavior of a parasite .
predation; preying or plundering.
relationships ; the way in which two things are connected .
terrestrial systems ; an ecosystem only found on land forms.
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Answer:
100 teragrams of nitrogen per year
Explanation:
Nitrogen fixation in Earth's ecosystems is defined as a process where by nitrogen in air is transformed into ammonia or other related nitrogenous compounds. Generally, atmospheric nitrogen is referred to as molecular dinitrogen and it is a nonreactive compound that is metabolically useless to all but a few microorganisms. This process is vital to life due to the fact that inorganic nitrogen compounds are needed for the biosynthesis of amino acids, protein, and all other nitrogen-containing organic compounds. Thus, the natural rate of nitrogen fixation in Earth's ecosystems is 100 tetragrams of nitrogen per year.
Three groups
gases, metals, metalliods/nonmetals
1) The metal which reduces the other compound is the one higher in the reactivity. So in this case it is 
.
2) The substance which brings about reduction while itself getting oxidised (that is losing electrons) is called a reducing agent. Here, $\mathrm{Zn}$ is the reducing agent and reduces Cobalt Oxide to Cobalt while itself getting oxidised to Zinc oxide.
