According to Newton's law of cooling, the heat energy lost per unit area of a body depends on the

I. Surface area
II. Nature of the surface
III. Temperature of the surface
IV. Excess temperature over the surroundings

In which of the following situations is photoelectric effect employed?

I. Automatic devices for switching on light at dusk
II. Automatic doors in departmental stores
III. Automatic devices for switching stores
IV. Automatic voltage stabilization