What tube carries air down to the windpipe?

The correct answer is the pharynx. The pharynx is a muscular tube that serves as a pathway for both air and food. It is located behind the nasal cavities and mouth, leading to the larynx and the esophagus. When air is inhaled, it travels through the nasal or oral passages into the pharynx, where it is then directed towards the larynx and subsequently down the trachea (windpipe). This structure plays a crucial role in respiration by allowing air to move efficiently from the nasal passages or mouth into the lower respiratory tract.

The other choices refer to different anatomical structures that do not directly carry air to the windpipe. The larynx is responsible for sound production and is located below the pharynx; the sinuses are air-filled spaces that help with breathing and resonance but do not transport air to the windpipe; and the alveoli are small air sacs in the lungs where gas exchange occurs but are not involved in the initial passage of air down to the windpipe.