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 most commonly responsible for traveler's diarrhea? 
Select and explain. 
A. E.coli, B. Rotavirus, C. Shigella, D. Ascaria
A: E.coli
Answer: A: E.coli 
Explanation: Traveler's Diarrhea is defined as three or more unformed stools in 24 hours passed by a traveler, commonly accompanied by abdominal cramps, nausea, and bloating. Its diagnosis does not imply a specific organism, but enterotoxigenic Escherichia coli is the most commonly isolated pathogen. Most cases are self-limited. Rotavirus causes watery diarrhea.