A 77-year-old man is a hospital inpatient admitted for exacerbation of his chronic obstructive pulmonary disease (COPD), and a respiratory therapist (RT) is assessing the client for the first time. Which of the following aspects of the patient's current state of health would be best characterized as a symptom rather than a sign?

a. The patient's oxygen saturation is 83% by pulse oxymetry.
b. The patient notes that he has increased work of breathing when lying supine.
c. The RT hears diminished breath sounds to the patient's lower lung fields bilaterally.
d. The patient's respiratory rate is 31 breaths/minute