Explanation:

Correct: Copper smelting plants, especially those utilizing pyrometallurgical techniques, produce a significant amount of slag, which can contain a variety of heavy metals. Over time, these metals, including arsenic, lead, and cadmium, can leach into the environment, potentially contaminating groundwater and soil, and posing a threat to human health and the environment.

Correct: Sulphur dioxide (SO2) is a primary by-product of the copper smelting process. When released into the atmosphere, SO2 can lead to acid rain, which can harm aquatic life, forests, and other vegetation. It can also combine with other airborne particles and create fine particulate matter, which poses respiratory risks to humans.

Incorrect: While carbon monoxide (CO) is a product of many industrial processes due to incomplete combustion, it is not typically associated with lethal releases specifically from copper smelting plants. The primary concerns with copper smelting are SO2 emissions and the potential environmental impact of slag.