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

Based on the observation that only male crickets produce a song, you hypothesize that a male's song is a form of communication to potential mates. you set up a simple experiment to test this hypothesis. in the laboratory, you place a male snowy tree cricket in enclosure a, which is adjacent to enclosureb. in enclosure b, you place other insects, one at a time, and observe their responses to the male's song. the enclosures are designed so that the two insects being tested cannot see or smell each other, but sound is transmitted from enclosure a to enclosureb.