causes dieses in cells, but the cannot be cured with antibodies (fill in the blank, this is a question about viruses btw)

b. the carbon cycle ensures living things have a source of hydrogen

helicase, i think thats it

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.