Which industrial facility is most likely to produce sludge and heavy-metal wastes?

A. A leather tannery

B. A medical center

C. A nuclear power plant

D. A steel-smelting facility

The right answer is: D. A steel-smelting facility

iron ore and coke are introduced into the blast furnace from above, the heat causing the combustion of the coke and the removal of the contaminating chemical elements. The iron is then loaded with carbon during its descent and turns into cast iron, which must then be separated from a mixture of sludge waste containing a heavy metal.