Why Might It Be More Difficult to Prepare a Vaccine

Why might it be more difficult to prepare a vaccine against noroviruses than against rotaviruses?

A) We haven't been able to culture noroviruses in a lab setting yet.Without a starting culture,we can't create a vaccine.
B) Noroviruses are RNA viruses,where rotaviruses are DNA viruses.RNA viruses mutate far more easily than DNA viruses,so we COULD make a vaccine,but it would be rendered useless fairly quickly as the virus mutates.
C) We lack a proper culturing method for large-scale production of target cells for norovirus,whereas we have such a system for the target cells of rotaviruses.Without a system to get large numbers of target cells,we can't produce a vaccine.
D) Norovirus is much more infectious than rotavirus.As such,it's much harder to work with safety.This makes production of a vaccine too dangerous and unpredictable.