a) $\underset{AA}{\rightarrow}$, $\underset{AB}{\rightarrow}$, $\underset{AC}{\rightarrow}$, $\underset{AD}{\rightarrow}$, $\underset{AE}{\rightarrow}$
b) $\underset{AB}{\rightarrow}$, $\underset{BB}{\rightarrow}$,$\underset{CB}{\rightarrow}$, $\underset{DB}{\rightarrow}$, $\underset{EB}{\rightarrow}$