The answer is A. A leather tannery. A tannery produces a sludge as a byproduct that is high in various toxic elements including chromium, a heavy metal that is highly toxic. Other pollutants produced include sodium, calcium, sulfur, magnesium, phosphate, copper, tin, lead, arsenic cadmium and zinc. Tannery effluent is an extremely toxic pollutant that could cause great harm if released into the environment, and for that reason, the tannery industry needs to be highly regulated and managed.