How Do We Know That It Is a Toxin That

How do we know that it is a toxin that Vibrio produces that causes the symptoms of cholera?

A) There's nothing else that could cause symptoms so severe.
B) All pathogenic bacteria produce toxins; that's why they're pathogenic.
C) An antitoxin can be used to treat the disease.
D) Experiments with people ingesting the toxin itself result in the same symptoms.
E) Experiments with mice ingesting the toxin itself result in the same symptoms.