We use AES$-$CBC to encrypt a message $\backslash ($ M $\backslash )$ and get two ciphertext blocks $($in hexadecimal format$)$:

$```$

The initialization vector $($IV$)$ is

$36$ f$56$a $3$b $643749$ a$2$ d$4256$a $7$b $62$ b$17$a $45$

The first ciphertext block is

ec $947$f ee $7$b $555313423283$ e$7$ f$1$ c$7$ cd $6$b

The second ciphertext block is

$82$ e$31$f $08$ d$402$ a$088647358626356$ b$7$

$```$

If we use AES$-$ECB to decrypt the two ciphertext blocks, we obtain:

$```$

e$578$ a$2$ ef $366$b $2$d e$1$ a$373$ b$73$f $6$d $78824$a

$0$c $810$e $2730764$a $3$a $8$b ef f$9$ cf c$563$ b$6$ f$1$

$```$

$($a$)$ Due to transmission error, the first byte of the first ciphertext block is changed from $\text{'}$ec$\text{'}$ to $\text{'}6$ b $\text{'}.$ We decrypt the ciphertext using AESCBC, what are the first two bytes of the second plaintext block after decryption?

$($b$)$ Due to transmission error, the first byte of the IV was changed from $\text{'}36\text{'}$ to $\text{'}45\text{'},$ but there is no error in the ciphertext blocks. We decrypt the ciphertext using AES$-$CBC$,$ what are the first two bytes of the first plaintext block after decryption? What are the first two bytes of the second plaintext block after decryption?