Why Is a "Four-Eyed" Dog with a Spot Above Each

Why is a "four-eyed" dog with a spot above each eye used in a Zoroastrian funeral ritual?

A) To frighten evil spirits into fleeing
B) To purify the soul of the dead person
C) To act as a guard to ensure the uninterrupted completion of the funeral ritual
D) To remove obstacles to ensure the reincarnation of the dead person