Question

what causes pneumonia?
an antigen
a bacterial infection of the lungs.
a viral infection

Explanation:

Pneumonia is an inflammatory condition of the lungs. It can be caused by various pathogens. A bacterial infection of the lungs is a common cause of pneumonia. Antigens are substances that can trigger an immune response but are not the direct cause of pneumonia. While viral infections can also lead to pneumonia, the most accurate and direct answer among the given options for what causes pneumonia (as a general statement considering common etiologies) is a bacterial infection of the lungs.

Answer:

A bacterial infection of the lungs.