Final answer:
Apollo could not take back the gift of prophecy from Cassandra because it was a permanent gift. After gifting it to her, he cursed her to ensure that no one would believe her prophecies.
Step-by-step explanation:
The reason Apollo couldn't take his gift of prophecy back from Cassandra is because it was a permanent gift.
According to mythology, gods and goddesses would bestow gifts upon humans or other beings, which were irreversible once given.
In Cassandra's case, Apollo granted her the gift of prophecy, but when she did not reciprocate his love, he cursed her so that her prophecies would never be believed by anyone.
This myth illustrates the theme of the irreversible nature of divine gifts and the complexities of the relationships between gods and mortals.