Why might the lack of an unpleasant odor in a manhole indicate danger?

The correct choice indicates that some dangerous gases have no odor. This is significant in an environmental compliance context because many hazardous gases, such as carbon monoxide or methane, can be colorless and odorless. When workers or personnel do not detect any unpleasant odors emanating from a manhole, they might mistakenly assume there is no risk present. However, the absence of smell does not equate to safety, as odorless gases could still be present at dangerous levels and pose significant health risks or lead to explosive conditions. Understanding that the absence of smell does not guarantee a lack of danger is crucial for maintaining safety and compliance in handling such environments.