It's normal for any active volcano to be monitored lest it erupts and brings disastrous damages on the nearby settlements. Which one among the

It's normal for any active volcano to be monitored lest it erupts and brings disastrous damages on the nearby settlements.

Which one among the following can signal that a volcano is about to erupt?

A.     When the number of shallow earthquakes from beneath the volcano suddenly start to increase.

B.     When there's an abnormally heavy thunderstorms over the volcanic mountain.

C.      A noticeable disappearance of migratory birds from the area.

D.     When the water whose source is near the volcano start to taste a bit of fluorine.