Drug X, which kills on contact the bacteria that cause the infection, is administered to patients by means of an aerosol inhaler.

Which of the following, if true, contributes most to explaining the change in drug X's effectiveness during the course of the infection?

(A) Symptoms of the infection usually become evident during the first 48 hours following infection.
(B) Most patients with lung infections say they prefer aerosol inhalers to other means of administering antibacterial drugs.
(C) In most patients taking drug X, the dosage administered is increased slightly each week until symptoms disappear.
(D) In patients who have the infection, the ability to inhale becomes increasingly impaired beginning in the second week after the onset of symptoms.
(E) Drug X is not administered to any patient who shows signs of suffering from secondary infections.