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
Bacillus used to test the efficacy of Sterilization by Autoclave is? 
Select and explain. 
A. Bacillus subtilis., B. Bacillus pumilus, C. Bacillus stearothermophilus, D. Coxiella burnetii
C: Bacillus stearothermophilus
Answer: C: Bacillus stearothermophilus 
Explanation: Various bacilli are used to check the efficacy of sterilization methods are:

Autoclave- Bacillus stearothermophilus
Hot air oven- Bacillus subtilis subspecies niger
Ionising radiation- Bacillus pumilus