Helicobacter Pylori Is Now Known to Cause Stomach and Duodenal

Helicobacter pylori is now known to cause stomach and duodenal ulcers.Marshall's drastic experimental ingestion of this microbe was required to prove beyond doubt that this was the causative agent.Why do you imagine that his experiment was necessary? (Select all that apply)

A) The hostile stomach pH was regarded as a sterile environment
B) Prevailing medical opinion was that stomach ulcers were caused by viruses
C) Cause and effect in pathogenesis always requires human experimentation
D) Antibiotic therapies had been tried in the past and shown ineffective
E) Medical opinion related ulcers to stress,diet and smoking