Which of the following statements about cad cells is true?

The statement that cad cells can become dirty due to soot is correct. Cadmium cells, or cad cells, are essential components in the operation of oil burners, as they serve as safety devices that detect flame presence. Over time, these cells can accumulate soot and other residues generated from incomplete combustion, which can hinder their performance. This buildup can prevent the cad cell from accurately detecting the flame, leading to operational issues such as the burner not igniting or shutting down due to perceived flame failure. Regular maintenance that includes cleaning or replacing cad cells is vital for ensuring that the oil burner operates safely and efficiently.

Other statements do not align with how cad cells function; they do not operate at peak performance all the time, they are not affected by oil levels, and they certainly do react to changes in light as that is their primary function - to detect the presence of flame.