The fact that the Moon's appearance changes from a full circle to a thin sliver demonstrates that the Moon...

...produces its own light source.

...is a flat disk, not a sphere.

...reflects light from the Sun.

...is moving into and out of Earth's shadow.

A student observes that the Moon appears as a thin crescent one night and nearly full two weeks later. What is the fundamental reason for this change in the Moon's appearance?

The Moon is physically shrinking and growing over time.

Clouds in Earth's atmosphere periodically block parts of the Moon from view.

The Moon's rocky surface changes its shape in a monthly cycle.

We are seeing different amounts of the Moon's sunlit side as it orbits the Earth.

The Moon's actual, physical shape changes in a predictable cycle.

The Moon generates its own light, which it turns on and off over the month.

The Moon does not change its spherical shape; only the illuminated portion we see from Earth changes.

The amount of light the Sun emits varies, causing the Moon to appear brighter or dimmer.

A large canyon that encircles the entire Moon.

The edge of Earth's shadow falling directly onto the Moon.

The sharp contrast between the side of the Moon lit by the Sun and the side in darkness.

A chemical reaction on the lunar surface that occurs at the boundary of day and night.