Question

select the correct answer from each drop - down menu.
extreme in the center of nebulae causes hydrogen atoms to shed their electrons and collide with one another. the of the collisions cause the smaller, bare nuclei to fuse into large helium nuclei.
expansion
heat
density
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Explanation:

In the center of nebulae, extreme heat provides the energy for hydrogen atoms to shed electrons. The force (energy from heat - related to high - energy collisions) of these collisions enables nuclear fusion of hydrogen nuclei into helium nuclei.

Answer:

First blank: heat; Second blank: energy (or force, as the context implies the driving factor of the collisions which is related to the energy from the high - heat environment causing the collisions to have enough 'oomph' for fusion).