<qa>  What is the most common pattern of oxygen therapy in mild AECOPD?For AECOPD patients, most of patients dont need respiratory support, only 8 patients ventilated by noninvasive and invasive positive pressure ventilation. Nasal cannula is the most common pattern of oxygen therapy in mild AECOPD. As a low flow oxygen therapy, nasal cannula has many disadvantages: discomfort, low humidity and unstable fraction of inspired oxygenFiO2 in inspiratory gas. Therefore, we design a RCT to explore whether HFNC would be better than nasal cannula to prevent the aggravation of respiratory failure and endotracheal intubation in mild AECOPD. 