| LIST I | LIST II |
|---|---|
| (A) Unification | (I) Variable can be done with a constant, another variable, or a function. |
| (B) Deep backtracking | (II) The entire conjunctive goal is executed. |
| (C) Forward movement | (III) Choose subgoal with possible unifier. |
| (D) Shallow backtracking | (IV) Previous subgoal to find alternative solutions. |
Consider the grammar $S \rightarrow aSa \mid bSb \mid a \mid b$. Which one of the following options correctly characterizes the language generated by the given grammar over the alphabet {a,b}