How Do Volcanologists Predict Volcanic Eruptions

How do Volcanologists predict volcanic eruptions?

A) They monitor gases emitted by a volcano, which increase in concentration before a volcanic eruption.
B) They monitor earthquakes, which increase in frequency before a volcanic eruption.
C) They watch the shape of a volcano, which increases in steepness before a volcanic eruption.
D) They use satellites to predict eruptions.
E) All of the above.