Identify the structure indicated by the X in the image below.

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A. It is lined with simple squamous epithelium: The trachea is lined with pseudostratified ciliated columnar epithelium, not simple squamous
B. It contains no air-filtering mechanisms: the trachea has mucus and ciliated cells that trap and move particles (air-filtering mechanisms present)
C. It is comprised of C-shaped cartilage rings: The trachea is supported anteriorly by C-shaped hyaline cartilage rings that keep the airway open
D. It is a passageway for food and for air: The trachea is only an air passageway; the esophagus transports food

A. Antigens in solution: cytotoxic T cells (CD8⁺) recognize antigen presented on MHC I of cells, not free/soluble antigens in plasma.
B. Cancer cells and virally infected cells: cytotoxic T cells detect abnormal peptides on MHC I and kill virally infected or neoplastically transformed (cancer) host cells.
C. Antigen-presenting cells: Partially misleading -professional APCs present antigen on MHC II to helper T cells; cytotoxic T cells usually target infected or abnormal host cells (though APCs can present antigen on MHC I and theoretically be targeted).
D. Helper T cells: helper T cells (CD4⁺) are immune coordinators, not the usual targets of cytotoxic T cells.