Why are muscle and fat levels the only factors influencing flexibility impacted by personal decisions?
a. muscle and fat levels are mostly determined by heredity.
b. age, gender, and joint structure cannot be changed.
c. personal decisions have significant impacts on age and gender.
d. flexibility is not dependent upon muscle and fat levels.

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.