This is because the encryption scheme in use adheres to:

An attacker captures the encrypted communication between two parties for a week, but is unable to decrypt the messages. The attacker then compromises the
session key during one exchange and successfully compromises a single message. The attacker plans to use this key to decrypt previously captured and future
communications, but is unable to. This is because the encryption scheme in use adheres to:

A.
Asymmetric encryption

B.
Out-of-band key exchange

C.
Perfect forward secrecy

D.
Secure key escrow