Answer: B
Explanation:
Ammonia is so toxic that it can be transported and excreted only in large volumes of very dilute solution. As a result most terrestrial animals simply don't have access to sufficient water to routine excrete ammonia.
The main advantage of urea is its very low toxicity. Animals can transport urea in the circulatory system and store it safely in high concentrations. Also much less water is loss when a given quantity of nitrogen is excreted in a concentrated solution of urea than would be in a dilute solution of ammonia.
Atom is the basic element of all living things because it's small. It is the smallest constituent unit of matter which has the properties of a chemical element. They are very small. Sizes are about 100 or 100, 000 fm. They are unstable. They are composed of a nucleus and electrons.
Melanin
is
the
answer
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looking
for
