Read this excerpt from "Paul Revere's Ride"

Meanwhile, his friend, through alley and street

Wanders and watches with eager ears,

Till in the silence around him he hears

The muster of men at the barrack door,

The sound of arms, and the tramp of feet,

And the measured tread of the grenadiers

Marching down to their boats on the shore.

Which words is a key transitional word that indicates events happening simultaneously with other events?

AMeanwhile
Beager
CTill
Dgrenadiers