What part of the amino acid varies from one amino acid to another

the answer is na (sodium)

the population of unionid mussels in the hudson river decreased after zebra mussels were introduced. zebra mussels, a nonnative species, competed with the unionid mussels for food. the zebra mussels were able to outcompete the unionid mussels, allowing the zebra mussel population to increase dramatically over six years. the unionid mussel population decreased due to a lack of food.

hope this :

∠gdh and ∠edh on edg.

answer is c