I have plaintext P and ciphertext C, is it possible to find K key, by which P was encrypted. AES is used to encrypt.
This is called a known-plaintext attack.
Modern ciphers such as AES are not vulnerable to these attacks.
Depends how you define "Possible". It is generally understood, for the moment, to be an impractical task. That said, storing plaintext values with their encrypted counterparts is never recommended. especially if you do this for many sets of plaintext/ciphertext pairs, you increase the amount of information available to the attacker and weaken the encryption.
来源：https://stackoverflow.com/questions/6044701/is-it-possible-to-find-key-to-decrypt-ciphertext-by-having-the-original-plain-te