The diagrams illustrate the structures of two amino acids. In one, there are two amino $(-\text{NH}_2)$ groups, numbered 1 and 2. In the other, there are two carboxylic $(-\text{COOH})$ groups, numbered 3 and 4. A peptide bond is made between the two amino acids. Which groups take part in forming the peptide bond?
- A1 and 4
- B2 and 3
- C2 and 4
- D1 and 3