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.

rising temperatures because when water vapor in the air is cooled, it condenses. it turns back into a liquid and attaches to dust particles in the air. these very small water droplets group together to form clouds

the correct answer is - d. china has a large and growing population.

the fact that china has a large (the biggest) population, and it is still growing, doesn't hinder the agriculture production in the country. in fact, it can for increased production in the agriculture, since there's more labor force, as well the sheer mass of people produces lots of scientists that come up with new inventions as to how to improve this sector. nice example is the inner mongolia region in china, in general dominated by the gobi desert, but few scientist came up with a nice idea as to how to make the region productive by using plants that are very resistant to droughts, so lots of people have been employed, and the region is becoming greener and greener each year, and the production of food in this region is also increasing.