Answer:
They were unaware of Perseus approaching them on the bank and his attack surprised them.
Step-by-step explanation:
In the Greek myth of "Perseus and the Quest for Medusa's Head", the story revolves around Perseus's claiming the head of Medusa at the orders of the king. He was subjected to the dangerous mission as a present to the king who was to be married.
Perseus, with the help of the gods, was able to get to the weeds when he saw the three Gorgons sleeping soundly. Knowing that looking straight in the eye of a Gorgon will turn him to stone, he used the shield given by the Maidens, used it to see the reflection of the sleeping monsters, and successfully decapitated Medusa's head. By the time the other two sleeping Gorgons were able to realize what had happened, Perseus was already way ahead of them and invisible too, with the help of the Cap of Darkness.
Thus, the correct answer is that the Gorgons were unaware of Perseus approaching them while they were sleeping and his surprise attack.