Final answer:
Metallica's One is significant rock because it challenges traditional song structures by using alternating time signatures, tempo changes, and an unconventional song structure.
Step-by-step explanation:
The author argues that Metallica's One is significant rock because it challenges traditional song structures.
This can be seen in the composition's use of alternating time signatures and tempo changes, as well as its unconventional song structure that includes sections without a repeated chorus.
For example, the song starts with a slow and melodic guitar intro, followed by heavy and fast-paced verses, leading to a powerful and intense climax.