Final answer:
The two sources of water on Earth mentioned in the question are volcanic activity on Earth and comets from space, the correct answer being A. These sources have contributed to Earth's water through the planet's geological history.
Step-by-step explanation:
The sources of water on Earth are volcanic activity (on Earth), and comets (from space). So, the correct answer to the question is A. Volcanic activity, comets.
Water on Earth may have come from two primary sources: water that was locked into the rocky material that accreted to form Earth and water that was brought to Earth through impacts by asteroids and comets. These celestial bodies, which formed from the same cloud that created the planets, were rich in icy materials. As these objects bombarded the young Earth, they contributed significantly to the accumulation of water and various organic compounds, eventually aiding in the creation of Earth's atmosphere and oceans. This information aligns well with the current evidence suggesting a combination of internal sources, like volcanic activity, and external impact sources, such as comets and asteroids, having provided Earth with its water.