what is the greatest danger from volcanoes??
a. watery lava
b. mud flows
c. oxygen
d. bulging of the earth's crust
The greatest danger? Depends on where you are and which volcano. But I would have to certainly include in the top dangers the danger of heavier than air poisonous gas, which kills all in front of it.
http://en.wikipedia.org/wiki/Plinian_eruption