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: Ophthalmology
Ulcus serpens is caused by: 
Select and explain. 
A. Pneumococcus, B. Pseudomonas, C. Gonococcus, D. All of the above
A: Pneumococcus
Answer: A: Pneumococcus 
Explanation: Ans. Pneumococcus