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: 
Primary pollutants enter the air directly. Some are released by natural processes, like ash from volcanoes. Most are released by human activities. Carbon oxides are released when fossil fuels burn.


Question: What type of pollutants enter the air directly?
primary
Primary pollutants.