Final answer:
A rapid alternation between two notes is called a a. trill in music.
Step-by-step explanation:
Rapid alternation between two notes is called a a. trill. In music, a trill is a decorative musical technique where the performer rapidly alternates between two adjacent notes. It is commonly used in classical music, particularly in piano and string instrument compositions.
For example, in Franz Liszt's composition "La Campanella," there are multiple trills that add ornamentation and excitement to the piece.