Which of the following statements are correct about the maintenance factor?
I) It is the ratio of illumination under normal working condition to the total lumen given out by the lamp.
II) It is the ratio of illumination under normal working condition to the illumination when the things are perfectly clean.
III) The accumulation of dust, dirt and smoke on the lamp decreases the maintenance factor.
IV) It is the ratio of illumination when the things are perfectly clean to the illumination under normal working condition.

Correct option is C

(II) It is the ratio of illumination under normal working condition to the illumination when the things are perfectly clean.
 Correct. The maintenance factor is calculated as the ratio of the actual illumination under normal working conditions (considering dust, dirt, and lamp aging) to the illumination when everything is new and perfectly clean.
(III) The accumulation of dust, dirt, and smoke on the lamp decreases the maintenance factor.
 Correct. Accumulation of dust, dirt, and smoke reduces the amount of light output, thus decreasing the maintenance factor.