Final answer:
The author introduces the brain and skeleton to highlight the important parts of the body, underlining not only their structural but also protective, hematopoietic, and storage roles. The correct option is OD. introduce readers to the important parts of the body.
Step-by-step explanation:
The author discusses the brain and the skeleton at the beginning of the selection to introduce readers to the important parts of the body. This introduction sets the stage for a comprehensive exploration of the human skeletal system and its various functions such as support, movement, protection of internal organs, production of blood cells, and storage and release of minerals and fat.
The description of how parts of the skeleton, including the brain case, protect vital organs like the brain and heart emphasizes the skeleton's protective function, making it clear it's not just a framework but also a shield for delicate tissues. Furthermore, by discussing the skeleton's ability to produce blood cells and its storage capabilities, readers are informed about the dynamic roles bones play beyond mere support and protection.