Why does the heart automatically adjust the flow of blood to match activity levels?
a. extreme activity damages blood cells and the body needs replacements.
b. the heart requires more oxygen when exercising.
c. it builds up a higher level of oxygen in the blood.
d. organs and muscles require more blood under stress.

Viruses use the host's machinery to make copies of themselves. however, some human viruses require a type of replication that humans do not normally have. for example, humans normally do not have the ability to convert rna into dna. how can these types of viruses infect humans, when human cells cannot perform a particular role that the virus requires? a) the virus causes mutations in the human cells, resulting in the formation of new enzymes that are capable of performing these roles. b) the viral genome codes for specialized enzymes not in the host. c) the virus infects only those cells and species that can perform all the replication roles necessary. d) viruses can stay in a quiescent state until the host cell evolves this ability.