I'm pretty sure it's the third option.
No more than five trophic levels are present in an ecosystem, most have only 4.
Elements [or radicals] with valency one are monovalent, those with valency two are divalent, and those with valency three are trivalent.
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What is the Valency of an Element / Radical?
Valency Divalent [2]
Element / Radical Oxide [Oxygen]
Symbol O
Ion O2-
Natural selection is one of the basic mechanisms of evolution during which those variations in the genotype that increase organism's chances of survival and procreation are preserved and multiplied from generation to generation at the expense of those that are less advantageous. In this case the following are postulates of natural selection; Individuals with certain traits are more likely to survive and reproduce, individuals in a population vary more likely to survive and reproduce, survival and reproductive success are variable among individuals in a population and lastly some traits differences are heritable.
