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: Microbiology
Inactivated microorganisms are used in the manufacture of which of the following: 
Select and explain. 
A. Salk vaccine, B. Tetanus toxoid, C. Sabin's oral vaccine, D. All of the above
A: Salk vaccine
Answer: A: Salk vaccine 
Explanation: None