If a Volcanologist Installs a Sensitive Seismometer Besides a Volcano

If a volcanologist installs a sensitive seismometer besides a volcano and it detects longer-period seismic waves this means that:

A) magma is still in the mantle zone
B) magma eruption is imminent
C) only gases will erupt from the volcano
D) no eruption is expected since short-period waves are the precursor for imminent eruption
E) none of these choices are correct,we still don't know the meaning of these signals