Prove that $\mathbb R_l$ $\times$ $\mathbb R_l$ given the box topology has a countable dense subset.

Question

Let $\mathbb R_l$ be $\mathbb R$ given the lower limit topology, i.e. the topology generated by $\{[a, b)\subseteq \mathbb R|a<b\}$.

Prove that $\mathbb R_l$$\times$$\mathbb R_l$ given the box topology has a countable dense subset.

I don't know how to show this, can anyone help me on this?