$P, Q, R$ 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 ________.

1. $\beta \alpha \:\text{and}\:\alpha \beta$
2. $\beta \beta \:\text{and}\:\alpha \alpha$
3. $\alpha \beta\:\text{and}\:\beta \beta$
4. $\beta\alpha \:\text{and}\:\beta \beta$

Given that, $P = \alpha \alpha,$ and  $Q = \alpha \beta$.

Now, the possibility for $R,S$ is $\beta \alpha , \beta \beta. \quad [\because \text{Uniquely coded}]$

So, the correct answer is $(D).$
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