Match the pairs in the following questions:$$\begin{array}{ll|ll} (a) & \text{Lexical analysis} & (p) & \text{DAG's} \\\hline (b) & \text{Code optimization} & (q) & \text{Syntax trees} \\\hline (c) & \text{Code generation} & (r) & \text{Push down automaton} \\\hline (d) & \text{Abelian groups} & (s) & \text{Finite automaton} \\ \end{array}$$