Why are very high temperatures and pressures required for fusion to occur? to generate the neutrons that are needed to break the nuclei
to overcome the repulsion between the protons in the nuclei that join
to maintain the proper conditions to keep the chain reaction going
to keep the uranium fuel separate from the control rods

Because the earth is not round ,but it may seem round when viewed from ,our planet but true some points earth is not round was an oblate spheroid -a square that it squashed as globe however is not even a perfect oblate spheroid.in fact isaac newton's first proposed that earth was not perfectly round but it was an oblate spheroid instead.that mean a square that is squashed as its poles and swollen at the