To make live attenuated poliovirus vaccine, why do scientists do multiple passages of the virus in animals and then cult

[answerhappygod]

To make live attenuated poliovirus vaccine, why do scientists do
multiple passages of the virus in animals and then cultured animal
cells? Choose the most appropriate one.
a. Because of the ethic issue, i.e. they cannot grow viruses in
human body
b. Because using animals and cultured cells are cheaper than
recruiting human volunteers
c. Because it’s easier to amplify the virus in animals and
cells
d. Because in a different species, the virus is more likely to
mutate and loss virulence