Mountains, hills, and valleys are considered landforms because they are natural features of the Earth's surface that have a distinct shape and elevation. They are formed by various geological processes over long periods of time.

Here's why each is considered a landform:

* Mountains: These are elevated landforms that rise significantly above the surrounding terrain. They are typically formed by tectonic plate collisions, volcanic activity, or erosion. Their distinct peaks, slopes, and valleys define them as a unique landform.

* Hills: These are smaller elevations of land compared to mountains. They often have gentler slopes and rounded summits. They can be formed by various processes like erosion, glacial activity, or volcanic activity. Their lower elevation and specific shape classify them as a distinct landform.

* Valleys: These are depressions or low areas between hills or mountains. They are typically formed by erosion from rivers, glaciers, or tectonic activity. Their lower elevation, elongated shape, and often the presence of water define them as a landform.

In essence, landforms are recognizable shapes on the Earth's surface created by natural processes. Mountains, hills, and valleys are all distinct features that contribute to the varied topography of the planet.