This compound was once used for executions in ancient Greece, and this compound forms in enclosed spaces when there is not enough oxygen to produce carbon dioxide. This compound is created as a byproduct in industrial processes like iron smelting and found naturally in the troposphere due to photochemical reactions. According to the ideal gas law, this compound is slightly less dense than air. For 10 points, name this chemical compound which blocks the space used by hemoglobin in the human body and is colorless, odorless and toxic to humans.
a) Carbon monoxide
b) Sulfur dioxide
c) Nitrogen oxide
d) Methane