This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Cystine is formed by ? 
Select and explain. 
A. Hydroxylation of cysteine molecule, B. Carboxylation of cysteine molecule, C. Peptide bond between two cysteine molecule, D. Disulfide bond between cysteine molecule
D: Disulfide bond between cysteine molecule
Answer: D: Disulfide bond between cysteine molecule 
Explanation: Ans. is 'd' i.e., Disulfide bond between cysteine molecule