Which is the best definition of metaphor?

A.
an unusual comparison that is meant to be funny; for example, "My uncle looks just like a sweet potato"

B.
a comparison between two things that are closely related; for example, "Gorillas are larger than chimpanzees"

C.
a direct comparison between two unlike things; for example, "My tongue is sandpaper"

D.
an indirect comparison that uses the words like or as; for example, "You're as pretty as a picture"