The Fungus Pneumocystis Jiroveci Is Found in the Lungs of Most

The fungus Pneumocystis jiroveci is found in the lungs of most people in low numbers, but in immunocompromised people it overgrows, resulting in severe respiratory problems. The fungus is best described as

A) resident microbiota.
B) transient microbiota.
C) a parasite.
D) a mutualist symbiotic partner.
E) both resident microbiota and opportunistic pathogen.