Analysis of test questions: lungs belong to the human respiratory system. According to "particulate matter with a diameter less than or equal to 2.5 microns in the atmosphere is called PM2.5, which is a fine particulate matter that can directly enter the lungs. PM2.5 is the main reason for the formation of "smog". The smog is rich in a lot of toxic and harmful substances, and it stays in the atmosphere for a long time and has a long transportation distance. " It can be known that PM 2.5 mainly harms the human respiratory system. Air enters the lungs and passes through the nasal cavity, pharynx, larynx, trachea and bronchus, but PM2.5 particles are tiny and are not easily blocked by organs such as nose. It can directly enter bronchi and alveoli, affecting gas exchange in the lungs and causing asthma and other diseases. After entering the lungs, it can be quickly absorbed by capillaries outside alveoli and distributed throughout the body through blood circulation. Particulate matter with a diameter less than or equal to 2.5 microns in the atmosphere can induce respiratory diseases. So I chose D.