known-plaintext attacks

[Jan Jansen]

Hi,

1. Is the AES-Encryption of a file by gnupg vulnerable to 
known-plaintext attacks ?
2. Does this depend on the lenght of the plaintext ?
	2.1 Is a strong Passphrase even safe, if the size of the known 
plaintext is just 1 Byte or even 10 GB ?
3. Is it possible for sombody, who does not know the strong Passphrase, 
to guess reliably, if the encrypted data contains a known file ?
4. Why or Why not (1.,2.,3 )?

Thanks.

JJ