Suppose two parties, Alice $($A$)$ and Bob $($B$),$ want to establish a common secret key $($D$-$H key$)$ using the Diffie$-$Hellman key exchange technique. They agree on $23$ as the modulus $($p$)$ and $5$ as the primitive root $($g$).$ Party A selects $6$ as her secret key $($a$),$ and party B chooses $9$ as his secret key $($b$).$ Calculate their Diffie$-$Hellman key.

Select one:

a$.21$

b$.13$

c$.11$

d$.9$