Practice question

Go with D — Two-thirds. Within Cardiovascular System: Heart, that statement lines up with the standard definition and the usual exam wording you’ll see. In plain terms, Two-thirds is the option that correctly names the structure, process, or principle asked for in Cardiovascular System: Heart. Holding that picture in mind makes Two-thirds feel natural rather than something you only memorise. Link the term to a real body example (organ, tissue, or ion flow) so the idea stays concrete under exam pressure. A short mental diagram helps — start from stimulus or structure, follow the pathway, and stop at the functional result described by Two-thirds.