Volcanic deposition and river erosion have both shaped parts of Idaho over geological time. is the correct choice. Basalt forms from cooled lava, while river erosion can cut deep gorges such as Hells Canyon.

Idaho contains high mountains, river-carved canyons, and volcanic terrain. Lava flows build rock layers, whereas rivers can erode them. Identifying the process behind a landform is more informative than memorizing the landmark name alone.