How did the hundred years' war affect the medieval structure of feudalism, in which vassals owed military service to the king?

a.
the vassal system became more important in winning battles and holding conquered territory for the king.

b.
the introduction of the longbow and cannon made the vassal system more efficient.

c.
the need for a larger, standing army with greater mobility made the vassal system obsolete.

d.
the vassals became commanders of mercenary armies paid by the king to fight.