Which feature of perm rods is directly linked to the degree of curl activation?

• A. Material
• B. Diameter
• C. Length
• D. Color

Answer: (B)

The diameter of perm rods directly determines how tight the resulting curl will be. A smaller diameter creates a smaller circle around which the hair is wrapped, producing a tighter, more activated curl. A larger diameter lets the hair wrap around a bigger circle, resulting in looser waves and less curl activation. The other features listed don’t control curl tightness in the same direct way: material, length, and color influence other aspects (such as strength of wrap, processing considerations, or cosmetic appearance) but not the fundamental degree of curl activation the way the rod’s diameter does.