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
Which of the following is used to test the efficiency of sterilization in an autoclave? 
Select and explain. 
A. Clostridium tetani, B. Bacillus stearothermophilus, C. Bacillus pumilus, D. Bacillus cereus
B: Bacillus stearothermophilus
Answer: B: Bacillus stearothermophilus 
Explanation: Autoclave: B. stearothermophillus
Hot air oven: Clostridium tetani nontoxigenic strain