A lamp is hanging along the axis of a circular table of radius r. At what height should the lamp be placed above the table, so that the illuminance at the edge of the table is \[\frac{1}{8}\] of that at its center [MP PET 2005]

• A. \[\frac{r}{2}\]
• B. \[\frac{r}{\sqrt{2}}\]
• C. \[\frac{r}{3}\]
• D. \[\frac{r}{\sqrt{3}}\]