To communicate between them, Anaïs and Bastien use an alphabet of only ten letters: 

a = 0, b = 1, c = 2, d = 3, e = 4, f = 5, g = 6, h = 7, i = 8, j = 9.

To encipher their messages, they decide to use the generalised Vigenère cipher on bigrams.

Bastien receives from Anaïs the ciphertext CCFC

Decipher the message knowing that the enciphering key is

A : 

 6  1 

 1  7 

B : 

 6 

 1 

Your answer will be a four letters word written in capital letters with no spaces.