Final answer:
To block out all light using polarizing filters, the filters must be perpendicular to each other.
Step-by-step explanation:
To block out all light using polarizing filters, the filters must be perpendicular to each other. When the axes of the first and second filters are aligned (parallel), all of the polarized light passed by the first filter is also passed by the second filter. However, when the axes are perpendicular, no light is passed by the second filter.