Amino acids are named after two of their functional groups: the amino group (NH2), which acts like a base, and the carboxyl group (COOH), which acts like an acid. Amino acids also contain a third functional group, often called the R group. In alanine, the R group is a methyl group (CH3). In serine, the R group is a methyl alcohol (–CH2OH). A molecule of alanine is shown next to a molecule of serine. Each molecular structure has an amino group (NH2) on the left side, and a carboxyl group (COOH on the right side. What functional groups will be joined together if alanine and serine molecules combine to form a single molecule? A. an amino group and a carboxyl group B. two amino groups C. an amino group and a methyl group D. a hydrogen atom and an methyl group