Let

A$=\{\backslash$epsi $,$ d c$,$ d c c$\}$ and B$=\{$c$,$ d c$,$ c c d$\}$

The concatenation of languages A and B$,$

A $$ B

is the set: $\{\backslash$epsi $,$ c$,$ d c$,$ c c d$,$ d c c$,$ d c d c$,$ d c c c$,$ d c c d d$,$ d c c d c$,$ d c c c c d$\}\{\backslash$epsi $,$ c$,$ d c$,$ c c d$,$ d d c$,$ d c d c$,$ d c c c$,$ d c c c d$,$ d c c d c$,$ d c c c c d$\}\{$c$,$ d c$,$ c c d$,$ d c c$,$ d c d c$,$ d c c c$,$ d c c c d$,$ d c c d c$,$ d c c c c d$\}\{$c$,$ d c$,$ c c d$,$ d c c$,$ d c d c$,$ d c c c d$,$ d c c d c$,$ d c c c c d$\}\{$c$,$ d c$,$ c c d$,$ d c c$,$ d c d c$,$ d c c c$,$ d c c d d$,$ d c c d c$,$ d c c c d d$\}$ None of the above.