Continuous heating and cooling of rock is a significant factor in soil formation. This process is primarily influenced by two agents - sunlight and rain.

1. Sunlight:
Sunlight plays a crucial role in the continuous heating and cooling of rocks. During the day, rocks absorb the heat from sunlight, causing them to expand. This expansion creates stress within the rocks. However, when the sun sets, the rocks start to cool down and contract, resulting in the release of stress. This continuous cycle of heating and cooling leads to the breakdown of rocks into smaller fragments, known as weathering.

2. Rain:
Rainwater also contributes to the continuous heating and cooling of rocks. When rain falls on hot rocks, it cools them down quickly. This rapid cooling causes the rocks to contract, resulting in the formation of cracks and fractures. Over time, these cracks become larger, leading to rock disintegration. Additionally, rainwater seeps into the cracks and fractures, and when it freezes, it expands. This expansion exerts pressure on the rocks, causing them to break apart further.

Combination of Sunlight and Rain:
The combination of sunlight and rain accelerates the process of rock weathering. The heating and cooling caused by sunlight weaken the rocks, making them more susceptible to the effects of rainwater. As rainwater seeps into the cracks and freezes, it widens the existing fractures, leading to the physical breakdown of rocks. This continuous process of heating and cooling, along with the action of rainwater, eventually breaks down the rocks into smaller particles, forming soil.

Other Options:
a) Air and water: While air and water can contribute to weathering processes, they are not responsible for the continuous heating and cooling of rocks, which is the primary focus of the question.
b) Plants and trees: Although plants and trees can indirectly contribute to soil formation through their root penetration and organic matter decay, they are not directly responsible for the continuous heating and cooling of rocks.
d) Plants and animals: Similar to the previous option, plants and animals play a role in soil formation through their interactions with the environment, but they are not responsible for the continuous heating and cooling of rocks.

In conclusion, the correct answer is option 'C' - sunlight and rain. These agents continuously heat and cool rocks, leading to their breakdown and the eventual formation of soil.

Air and water