Definition: In this task, you are given a multiple-choice question about healthcare. Answer the question based on your information and classify your answers into '1', '2', '3', and '4'.
Input: Question: Which of the following covalent bonds does not appear in proteins ?: 
 Options:  <1> Phosphodiester linkage. <2> Amide link. <3> O-glucosidic bond. <4> Disulfide bond. <5> Hydrogen bond.
