Mateo’s parents are thinking about investing money to help develop a new fusion power plant. Which issue is most important to consider before making the investment?

a)Fusion has never been controlled, so how have they succeeded in doing it?
b)The fuel is quite limited, so have they discovered a more common fuel?
c)Fusion has been generating electricity for decades, so why build another fusion power plant?
d)Fusion requires high temperatures and pressures, so how have they reduced the energy cost for it?