Most of the mass of an atom is located in the (nucleus), and most of the volume is taken up by the (electron clouds).
The ones in the parenthesis are your answers.
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Answer:
The same genes or slightly different versions of the same gene can be found on each chromosome in a pair. They form a line and split off bits of themselves, which they barter with one another. In sexual reproduction, crossing over is the first method that genes are shuffled to develop genetic variation.
the poly atomic molecule is H2
Out of the three you gave ecosystem is what I would call the most broad. An ecosystem includes living AND non-living things in an area
Community is all the LIVING things in an area (plants are included and I think bacteria is too, but I'm not sure)
Population is pretty narrow it just looks at specific species in one area. Population of mosquitoes doesn't include butterflies, if only looks at mosquitoes
Hope this helped!