Here's why:
* Hills and flats are different landforms: They have distinct characteristics and are not typically grouped together.
* Collective nouns are specific to a particular type of thing: For example, "a flock of birds" or "a herd of cattle."
Possible options to describe hills and flats together:
* Terrain: This is a general term for the physical features of a land area.
* Landscape: This term describes the overall appearance of a region, including hills and flats.
* Topography: This refers to the detailed features of a land surface, encompassing hills and flats.
* The land: This is a simple and general way to describe the combination of hills and flats.
You can also use more specific descriptions depending on the context:
* A chain of hills and a valley
* Rolling hills and flat meadows
* A rugged mountain range and its surrounding plains
Ultimately, the best way to describe a combination of hills and flats depends on the specific context and the level of detail you want to convey.