What are the important parts of the ciculatory system

the heart,lungs, and arteries, veins,

Explanation:

The system is responsible for the flow of blood, nutrients, oxygen and other gases, and as well as hormones to and from cells.

i think a) betsy can donate only blood to mark is correct because, mark and betsy have the same blood type. whereas nobody else has b+ blood so betsy can't donate to them.

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.