Which pair of statements best describes an essential amino acid?
it is an amino acid that contains peptide bonds. an example is proline.
it is an amino acid that contains nitrogen. an example is aspartic acid.
it is an amino acid that cannot be made by the body. it must be obtained from eating certain foods.
it is an amino acid that can be produced by the body. vitamin supplements maintain healthy levels

C

Explanation:

It is an amino acid that cannot be produced by the body. It must be obtained from eating certain foods.

correct answer is option a.

tropopause, stratopause and mesopause have all in common aspect that is point of maximum temperature in its layer of the atmosphere. all layers of atmosphere observe abrupt change in temperature. tropopause is the boundary between troposphere and stratosphere. stratopause is the boundary between stratosphere and mesosphere. mesopause is the boundary between mesosphere and thermosphere. these all boundaries are ranges between altitude of 0 to 80 km.

trachea, alveoli, lung capillaries, arteries