The characteristics of synthetic amino acids are different from the standard amino acids in 20, and the integration of these synthetic amino acids into proteins can systematically change the structure and biological characteristics of proteins. However, researchers have so far only been able to integrate one synthetic amino acid into one Protein, so you can only change one characteristic of a protein at a time. Which of the following is a prerequisite for the above discussion? ['Changing the structure of a protein can change its biological characteristics.', 'Researchers can only change the structure of one protein at a time.', 'A synthetic amino acid can only change one structure of a protein.', 'A synthetic amino acid can only change one characteristic of a protein.']