Let X be an arbitrary set and consider $\displaystyle d: X \times X \rightarrow R$ defined by $\displaystyle d(x,y) := \left \{\begin{array}{cc}0,&\mbox{ if } x = y \\1, & \mbox{ if } x \neq y \end{array}\right.$

I need to find $\displaystyle B_{1}(x)$ where $\displaystyle x \in X$. Don't know how... help?