What was the main reason for different behavior of gas and dust in the solar disk?

Correct answer: Difference in temperature and condensation temperature

The early solar disk contained materials that experienced different temperatures at different distances from the young Sun. As the disk cooled, substances condensed at the temperatures suitable for them. Refractory materials, including many rocks and metals, could condense in hotter inner regions, while volatile substances remained gaseous there and condensed farther away in cooler regions. This temperature gradient helps explain why inner planets are mainly rocky and outer planets contain much more gas and ice.

Option B is correct because differences in temperature and condensation temperatures produced different material behaviour. Rainfall on Earth, the Moon’s shadow, and human activity cannot explain the composition of planets forming in the solar disk. The answer should connect the changing temperature with the point at which different substances changed from gas into solid material. That process influenced the later composition of planets.