Which statement accurately describes a reason for the start of the crusades?

a.
saladin led a muslim army in an invasion of italy and sacked rome.

b.
muslim leaders asked westerners to them protect their centers of learning from attack by barbarians.

c.
the city of jerusalem had fallen under the control of jewish extremists.

d.
the byzantine emperor asked for from the pope to defend his realm from the seljuk turks.