Answer: C. Grade density of smoke
Ringelmann's scale is used to grade density of smoke.
The Ringelmann scale is a scale used for the determination of the apparent density or opacity of Smoke.
- French professor of agricultural engineering Maximilien Ringelmann of La Station d'Essais de Machines first developed the Ringelmann scale in Paris in 1888.
- The scale has 5 levels of density implied from a grid of black lines on a white surface which merge into known shades of grey if viewed from a distance.
- Shade 1 is slightly grey and is classified by air pollution boards as acceptable. It resembles to an opacity of 20%.
- Shades 2, 3, 4 and 5 resemble to opacities of 40%, 60%, 80% and 100% i.e. completely black and are treated to be "black smoke" by air pollution boards of most of the countries.
Hence, the correct option is C.
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