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C. production of white blood cells that can destroy disease-causing organisms

Explanation:

Production of blood cells, certain bones in the skeleton contain red bone marrow and the bone marrow produces red blood cells, white blood cells and platelets. The skeletal system does produce blood cells. White blood cells, red blood cells, and platelets are all produced in the bone marrow, which is a spongy tissue found in the middle of many bones of the skeletal system. These blood cells are produced by a special type of stem cell known as hemopoietic cells, which are found in the bone marrow.

the sugar-phosphate backbone forms the structural framework of nucleic acids, including dna and rna. this backbone is composed of alternating sugar and phosphate groups, and defines directionality of the molecule.