Which of the following are the PRIMARY muscles of respiration? (Select All that Apply.)

A. Intercostals: The intercostal muscles, including the external and internal intercostals, are primary muscles of respiration. They play a crucial role in expanding and contracting the thoracic cavity during inhalation and exhalation, facilitating airflow into and out of the lungs. B. Scalene: The scalene muscles assist in elevating the first two ribs during deep inhalation, making them accessory muscles of respiration rather than primary muscles. They support breathing but are not the main muscles involved in the normal respiratory cycle. C. Sternomastoid: The sternocleidomastoid muscles are also considered accessory muscles of respiration. They help elevate the sternum during forced inhalation, but they are not classified as primary muscles of respiration, which are more involved in the regular breathing process. D. Diaphragm: The diaphragm is the primary muscle of respiration and is crucial for normal breathing. It contracts and flattens during inhalation, increasing the volume of the thoracic cavity and allowing air to flow into the lungs. This muscle is essential for the majority of airflow during normal breathing. E. Pectoralis major: The pectoralis major is primarily involved in shoulder movement and is not a primary muscle of respiration. Although it can assist during forceful inhalation when the arms are raised, it does not function as a main muscle in the respiratory process.