Which feature of the Earth allows it to maintain large, stable bodies of liquid water on its surface over long periods of time?

Introduction / Context:
One of the defining characteristics of the Earth is the presence of large oceans, lakes, and rivers filled with liquid water. Liquid water is essential for life as we know it and depends on specific physical conditions. This question asks you to identify which feature of the Earth makes it possible to maintain bodies of liquid water for long periods, rather than having water permanently frozen as ice or boiled away as steam.

Given Data / Assumptions:
- Water remains liquid only within a certain temperature range near its freezing and boiling points, under normal atmospheric pressure.
- The Earth receives energy from the Sun, and its distance from the Sun helps set an overall temperature range.
- The options include physical features like mountains and tides, as well as processes like comet crashes and environmental conditions like moderate temperatures.
- We assume average surface conditions on Earth, not extreme local environments.

Concept / Approach:
Liquid water exists on the surface of a planet when temperatures are kept within a range where water neither freezes completely nor boils away. This depends on the planet's distance from its star, the composition of the atmosphere, and the presence of a greenhouse effect that moderates temperature. The phrase moderate temperatures captures the idea that Earth's climate generally stays within a suitable range to keep large amounts of water in liquid form. While mountains, tides, and rare comet impacts may influence water distribution or origin, they do not by themselves maintain water in the liquid state over billions of years.

Step-by-Step Solution:
Step 1: Focus on the key requirement in the question, maintaining bodies of water in a liquid state over long periods. Step 2: Evaluate option A, mountains, which can affect local climate and river systems but do not directly control global temperature suitable for liquid water. Step 3: Evaluate option B, tides, which are caused by gravitational forces and mainly move water, without determining whether it remains liquid. Step 4: Evaluate option C, moderate temperatures, which directly relate to keeping water between its freezing and boiling points. Step 5: Evaluate option D, comet crashes, which may have delivered water in the distant past but do not maintain its liquid state day to day. Step 6: Evaluate option E, strong magnetic storms, which are related to the magnetosphere and solar activity, not the temperature range for liquid water. Step 7: Conclude that moderate temperatures is the correct answer.

Verification / Alternative check:
To verify, consider other bodies in the solar system. Mars is colder and loses much of its surface water to ice or evaporation, while Venus is extremely hot and has no stable liquid water on its surface. Both differences are due to their temperature environments. Earth, by contrast, is in what is often called the habitable zone, where the balance of solar input and atmospheric greenhouse effect leads to moderate surface temperatures. This comparison supports the conclusion that moderate temperatures are the crucial factor in maintaining liquid water.

Why Other Options Are Wrong:
Mountains, option A, influence local rainfall and river patterns but are not responsible for the global temperature range that keeps water liquid.
Tides, option B, move water in oceans and seas and affect coastal ecosystems, but they do not determine whether the water is frozen, liquid, or gaseous.
Comet crashes, option D, may have contributed some water in the early history of the Earth, but they are rare events and do not continuously maintain the liquid state of water.
Strong magnetic storms, option E, are associated with solar wind interactions but are not the controlling factor for the long term surface temperature needed for liquid water.

Common Pitfalls:
A common mistake is to think of how water might have arrived on Earth, for example by comet impacts, and confuse this with the question of how it is kept liquid. Another pitfall is focusing on dramatic phenomena like tides and mountains without considering the more fundamental physical requirement of temperature. To avoid these errors, remember that the state of water depends mainly on temperature and pressure, and on Earth, moderate temperatures linked to its orbit and atmosphere are what allow large bodies of liquid water to exist.

Final Answer:
Moderate temperatures on Earth allow it to maintain large, stable bodies of liquid water on its surface over long periods of time.