How can we determine which story is a myth and which is a legend?
What are the differences between them?

A legend is a semi-true story, which has been passed on from person-to-person and has important meaning or symbolism for the culture in which it originates and is based on historic facts.A myth however is a story based on tradition , which has a deep symbolic meaning. A myth 'conveys a truth' to those who tell it and hear it, rather than necessarily recording a true event. They are often used to explain how something came about.

The way to tell the difference would be looking for fact and exaggeration. If you see these its most likely a legend. If there is a lot of symbolism or trying to explain the origins of something, it is most likely a myth.

i believe the answer is: b. mr. head had a long, tube-like face with a rounded open jaw and a short depressed nose.

this is because in the un-revised sentence it said that mr. heads face was long and tube-like, and he had a rounded open jaw. this is not a very grammatically correct because of how it seems to drag on with the and, replaced with, with, makes it sound more fluent in the description.