This is the variable group in an amino acid this part of an amino acid is what makes it unique

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."

sulfur dioxide

explanation: acid rain is caused by a chemical reaction that begins when compounds like sulfur dioxide and nitrogen oxides are released into the air. these substances can rise very high into the atmosphere, where they mix and react with water, oxygen, and other chemicals to form more acidic pollutants, known as acid rain.