The right answer is:
[Athena and Hermes] told him which one was Medusa and that was important, for she alone of the three could be killed; the other two were immortal.
Perseus is the Greek Hero who goes on adventure ans his objective, among others, is to kill a monster called Medusa Gorgon. She had a power to turn into stone jus with her look. So, no ordinary man were able to kill this monster, just a true Hero were able to do so.
