Which hypothesis was tested by Warren and Marshall during their research involving Helicobacter pylori bacteria and stomach ulcers?

A) Eating spicy food increases populations of H. pylori in the stomachs of people susceptible to ulcers.
B) The cause of many stomach ulcers is the bacterium H. pylori, not spicy food.
C) Stomach acid production is decreased by H. pylori bacteria in the stomachs of people with ulcers.
D) Acute stomach pain is often caused by H. pylori bacteria, not ulcers.