Read the following paragraph and answer the question. Give ONLY the answer, no other words or explanation.

For example:

Answer: <most likely answer, as short as possible>.

Paragraph: 
Courtesy of the CDC/Debora Cartagena. Cigarettes are a major source of chemical carcinogens .


Question: Cigarettes are a major source of what kind of chemical substances linked with cancer?
carcinogens
Chemical carcinogens.