Some regions of a polypeptide may coil or fold back on themselves. this is called and the coils or folds are held in place by (concept 5.4) a. tertiary structure covalent bonds b. primary structure covalent bonds c. tertiary structure hydrogen bonds d. secondary structure hydrogen bonds e. secondary structure peptide bonds

d. secondary structure ... hydrogen bonds

Explanation:

The secondary structure of a polypeptide corresponds to localized regions of orderly structure stabilized by hydrogen bonds between the backbone groups -NH and C = O and in which no hydrogen bonds involving the side chains participate. The secondary structure of the polypeptide is characterized by its ability to curl or fold, the folds being maintained by the polypeptide chains.

for a different answer set the answer would be because the oxygen in the water had depleted (apex)