TOEFL Listening: ETS-TOEFL听力机经 - P2Z71KP3W0L3IYC3P$

What does the professor say about the recurrence interval? A. It is the most reliable predictor of a volcano's next eruption. B. It tends to increase over the course of a volcano's existence. C. It can be estimated by the depth of a volcano's magma chamber. D. It can vary widely from volcano.