Mount St. Helens in Washington State erupted in 2004 and did not quiet again until 2006. Here are some of the observations scientists recorded in the fall of 2004.

Sept. 23-25: Earthquakes began, too small to be felt by most people at first.
Sept. 29: Scientists observed that deformation—a slight change of shape—had taken place on part of the crater floor.
Oct. 1: Steam, ash, and gases started escaping from the volcano.
Oct. 2: The first long, continuous tremor occurred.
Oct. 4: The bottom of the crater floor rose by tens of meters.
Oct. 7: The volcano was emitting gas at a rate of 2,400 tons per day of CO2, 100 tons per day of sulfur dioxide, and 10 tons per day of H2S.
Oct. 11: The first magma reached the surface.
How is information like the above being used to protect people from volcanoes?
A.
Scientists monitor active volcanoes for signs the the volcano is going to erupt in order to evacuate threatened areas in time.
B.
Scientists monitor active volcanoes so that they can relieve the pressure in them before the volcano is forced to erupt.
C.
Scientists use the data collected to learn how volcanoes work so that they can prevent or delay eruptions.
D.
Scientists use the data collected to learn how eruptions work so that they can prevent all deaths and property damage due to lava and ash