According to the nebular hypothesis, when did nuclear fusion begin?

after the planets formed.
when the sun was 5 billion years old.
when the nebula first exploded outward into space.
when particles were pulled together so tightly they combined.

When particles were pulled together so tightly they combined.

The correct answer is "When particles were pulled together so tightly they combined".

Explanation:

The nebular hypothesis is the most accepted hypothesis about how the Solar System was created. According to this hypothesis a nebula collapsed and created the particles that later formed the Solar System. First the gravitation force put the particles close to each other. Later, nuclear fusion began when particles were pulled together so tightly they combined and sufficient temperature and pressure was achieved.

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