Question $1$: Why is CFB mode probably not a good selection for a full disk encryption algorithm?

Question $2$: In AES what is the mix columns operation, and what is its purpose?

Question $3$: Why are encryption and decryption the same operation for stream ciphers?

Question $4$: Answer this question for CBC mode: If a single bit is corrupted $($flipped$)$ in one block of the ciphertext when the mode is used, what will the effect be for the message recipient? Assume the message is three blocks long and the corruption happens in the $2$nd block.

Question $5$: Suppose someone says "Triple encryption only doubles the security level." What do they mean?

Question $6$: Why do you think AES offers $3$ key sizes?

Bonus Question: Consider $\backslash ($ G$=\backslash$left$\backslash \{$x$^\{$k$\}$: k $\backslash$in $\backslash$mathbb$\{$Z$\}\backslash$right$\backslash \}\backslash ),$ in other words "the powers of $\backslash ($ x $\backslash )"$ together with the multiplication operation. Is this a group? Check the axioms one by one.