Let $\backslash$Sigma $=\{$a$,$b$\}.$ What does the following TM do$?$

q$0$ a b R q$0$

q$0$ b a R q$0$

q$0\_\_$ L q$1$

q$1$ a b L q$1$

q$1$ b a L q$1$

q$1\_\_$ S halt$-$accept

Choice $1$ of $4$:It enters an infinite loop. Choice $2$ of $4$:It checks whether the string consists of alternating $\text{'}$a$\text{'}$s and $\text{'}$b$\text{'}$s$.$ Choice $3$ of $4$:It moves right to the end of the string, switching a$\text{'}$s to b$\text{'}$s and b$\text{'}$s to a$\text{'}$s$.$ It then moves to the start of the string switching them back, and then it accepts. Choice $4$ of $4$:It moves right to the end of the string and then moves to the start of the string, all the while not changing the string on the tape, and then it accepts.