Why is there a concern about copper smelting plants? 1. They may release lethal quantities of carbon monoxide into environment. 2. The copper slag can cause the leaching of some heavy metals into environment. 3. They may release sulphur dioxide as a pollutant. Select the correct answer using the code given below.

Copper smelting plants are significant environmental concerns because they release large amounts of sulphur dioxide ($SO_2$) from sulfide ores and produce slag that leaches toxic heavy metals like arsenic and lead into soil and water. Statement 1 is incorrect because "lethal quantities" of carbon monoxide is an exaggeration; while CO is a byproduct of incomplete combustion, $SO_2$ is the primary gaseous pollutant of concern in copper metallurgy. This question tests the core concept of industrial pollutants and the specific environmental hazards associated with non-ferrous metal smelting processes.