$ \text{Given: Fix } x_0 \in \mathbb{R}. $
$ \text{Show: } \{ (x_0,y) | \ y\in \mathbb{Q} \} \in \mathcal{B} \text{, the Borels on } \mathbb{R}^2. $
$ \text{Proof: } \{ (x_0,y) | \ y\in \mathbb{Q} \} \text{ is a countable union of closed sets, thus } F_\sigma. $
