What's the main difference between a metaphor and a simile?

a. a simile uses "like" or "as" to make comparisons and a metaphor doesn't.
b. a simile is an implied comparison and a metaphor is a direct one.
c. a simile compares two things by using the word "like" and a metaphor compares more than two things.
d. a simile is a type of figurative language, and a metaphor isn't.