Which of these is a source for particulate matter in the air A. automobiles B. dust C. fires D. all of the above

## Step 1<br />Particulate matter refers to tiny particles or droplets in the air that are 2.5 micrometers or less in width, which is about 1/30th the width of a human hair. These particles can be made up of a variety of substances, including organic and inorganic compounds. They can be emitted from various sources, including natural and human-made sources.<br /><br />## Step 2<br />Automobiles are a significant source of particulate matter. They emit exhaust gases, which contain a variety of particulate matter. These particles can be harmful to human health and the environment.<br /><br />## Step 3<br />Dust is another source of particulate matter. It can be generated from various sources, including soil, pollen, and other natural materials. Dust can be carried by the wind and can settle in the air, contributing to the particulate matter content.<br /><br />## Step 4<br />Fires are also a source of particulate matter. When organic materials such as wood, paper, and other combustible materials are burned, they produce particulate matter. This particulate matter can be harmful to human health and the environment.<br /><br />## Step 5<br />Given the above explanations, it is clear that all of the options listed (automobiles, dust, and fires) are sources of particulate matter in the air. Therefore, the correct answer is D, all of the above.