Final answer:
The pharynx is the common space used by both the respiratory and digestive systems, as it serves to conduct air to the larynx and food to the esophagus. The correct answer to the question is a. pharynx.
Step-by-step explanation:
The common space used by both the respiratory and digestive systems is the pharynx. The pharynx is a tube-like structure that connects the nasal cavity and the back of the mouth to other structures lower in the throat, including the esophagus and the larynx.
The pharynx serves a dual function: it is part of the respiratory system as it conducts air from the nasal cavity to the larynx, and it is part of the digestive system as it allows swallowed food to pass from the oral cavity to the esophagus. During the process of swallowing, a flap of tissue called the epiglottis covers the opening to the larynx to prevent food or liquids from entering the airways. Therefore, the correct answer to the original question is a. pharynx.