$\text{P, Q, R and S}$ are to be uniquely coded using $\alpha$ and $\beta$. If $P$ is coded as $\alpha\alpha$ and $Q$ as $\alpha\beta,$ then $R$ and $S$, respectively, can be coded as ___________.
- $\beta\alpha$ and $\alpha\beta$
- $\beta\beta$ and $\alpha\alpha$
- $\alpha\beta$ and $\beta\beta$
- $\beta\alpha$ and $\beta\beta$