What best describes the characteristics of a good countermelody in traditional two-voice counterpoint?

A good countermelody in traditional two-voice counterpoint is characterized by sustained notes that provide a contrast to rhythmically active portions of the primary melody. This contrast enhances the overall texture and depth of the music, allowing both voices to complement each other perfectly. By having one voice sustain notes while the other plays rhythmically active elements, it creates a dynamic interplay that enriches the listening experience.

In contrast, a countermelody that incorporates fast tempo or fast notes might compete with the primary melody instead of complementing it. Similarly, if both voices moved in the same rhythm, they would lack the necessary contrast that creates a distinct and effective countermelody. Lastly, while simple harmonies can be effective in some contexts, they do not specifically define a good countermelody within the framework of traditional counterpoint, which often thrives on interplay and contrast between the melodic lines.