What Phrase Best Characterizes Microbiota of the Respiratory Tract

What phrase best characterizes microbiota of the respiratory tract?

A) respiratory microbiota are well adapted to their location and can resist removal or destruction.
B) less than 100 species of microorganisms reside in the respiratory tract.
C) skin microbiota are also residents of the lower respiratory tract
D) Streptococcus pneumoniae may be considered members of lower respiratory tract microbiota