Here's why:

* Crystalline solids have a highly ordered, repeating arrangement of their molecules. This structure gives them a defined melting point and a predictable crystal shape.

* Amorphous solids lack this long-range order in their molecular arrangement. They have a more random structure, which results in a gradual softening point instead of a distinct melting point.

Dry ice, which is solid carbon dioxide (CO₂), has a distinct crystal structure (cubic) and a well-defined melting point (-78.5°C or -109.3°F). Therefore, it fits the definition of a crystalline solid, not an amorphous solid.