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: Medicine
A 70 years old nursing home patient refused influenza vaccine and develops influenza died of acute pneumonic attack one week after influenza. Most common cause of acute post influenza pneumonia is? 
Select and explain. 
A. Measles, B. Staphylococcus aureus, C. Cytomegalovirus, D. Legionella
B: Staphylococcus aureus
Answer: B: Staphylococcus aureus 
Explanation: None