Question

which is a precursor to a geological event?
steam rising from a volcanos mouth before an eruption
a car driving over a bridge before an earthquake

Explanation:

A precursor is an event or phenomenon that indicates a larger event is likely to occur. In geology, steam rising from a volcano's mouth before an eruption is a known precursor as it can signal magma movement and gas release. A car driving over a bridge has no geological connection to an earthquake.

Answer:

steam rising from a volcano's mouth before an eruption