Toxic vapors could be generated as a __________ of an __________ or an abnormal chemical reaction.

• A. product | unexpected
• B. product or by-product | unexpected
• C. by-product | unexpected
• D. product | expected

Answer: (B)

Toxic vapors can be generated as a product or by-product when a chemical reaction goes awry—becomes unexpected or abnormal. This means the vapors might be the main result of the reaction or just an unintended side product, and they arise from reactions that don’t proceed as planned. That dual possibility (product or by-product) together with the idea of an unexpected/abnormal reaction best captures the hazardous scenarios, since normal, well-controlled reactions are less likely to produce unforeseen toxic vapors.