[Real Analysis] Show that $B$ is countable.

$B = \{f :{\displaystyle \mathbb {R}}?{\displaystyle \mathbb {R}}$ such that $f(x) =$$\sum_{n=1}^{N} a_nx^n$, $N ?{\displaystyle \mathbb {N}}, a_1, . . . , a_N ?{\displaystyle \mathbb {N}}\}$.
Show that $B$ is countable.

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