\textbf{155-} Given $P(X = x) = \dfrac{\dbinom{5}{x}\dbinom{4}{r-x}}{a}$\,; $x = 0, 1, 2, 3$, for which value of $a$ is this a probability function?
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(1) $48$ \hfill (2) $56$ \hfill (3) $64$ \hfill (4) $84$
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