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

Explanation:

to overcome the repulsion between the protons in the nuclei that join

answer;

d. are made mostly of liquid

explanation;

-the general   composition and structure planets includes; they have a thick gas and liquid layer covering a small,solid core. mostly the gases are hydrogen and helium atmosphere and layers of liquid hydrogen. for instance neptune and uranus are examples of the outer planets, they have a core covered by ice and ammonia.

-neptune's atmosphere is hydrogen and helium with a bit of methane so there is no oxygen to breath and the interior is made up of frozen water and ammonia with a rock and iron core. thus, there is no liquid water or solid surface to survive.