The substance that reduces surface tension in the alveoli is:

A. Hypercapnia refers to elevated levels of carbon dioxide (CO₂) in the blood, which can occur due to respiratory problems or insufficient ventilation.
B. Hypoxemia is a condition where there is low oxygen in the blood, not related to carbon dioxide levels.
C. Hypocapnia refers to low levels of carbon dioxide in the blood, often caused by hyperventilation.
D. Hypoxia refers to low oxygen levels in the tissues, not the blood levels of carbon dioxide.

A. Crackles are discontinuous, short popping sounds that are often heard during inhalation and are typically associated with conditions like pneumonia or pulmonary edema, not asthma.
B. Wheezes are continuous, high-pitched sounds often heard during exhalation and are commonly associated with asthma or other obstructive airway conditions. These sounds are caused by the narrowing of the airways.
C. Stridor is a harsh, high-pitched sound often heard during inhalation and indicates upper airway obstruction, such as in croup or a foreign body in the airway.
D. Rhonchi are low-pitched, gurgling sounds that are often associated with blockages in the larger airways (such as mucus) and are generally heard during both inhalation and exhalation, but they are not typically high-pitched like wheezes.