**Core Concept**
The Angle's Classification of occlusion and its associated indices, such as the Average Perioral Divergence Index (APDI), are crucial in orthodontics and dental anatomy. APDI is a measure used to evaluate the occlusal relationship between the upper and lower teeth.

**Why the Correct Answer is Right**
The mean value of APDI in normal occlusion is typically around 45-50 degrees. This value indicates a harmonious relationship between the upper and lower teeth, allowing for proper distribution of forces during mastication. The APDI value is influenced by the inclination of the mandibular incisors and the shape of the dental arches.

**Why Each Wrong Option is Incorrect**
**Option A:** A value less than 30 degrees would indicate a Class I malocclusion, where the upper teeth overlap the lower teeth, and is not representative of normal occlusion.
**Option B:** A value greater than 60 degrees would indicate a Class III malocclusion, where the lower teeth overlap the upper teeth, and is also not representative of normal occlusion.
**Option C:** A value of 0 degrees would indicate a perfect vertical overlap, which is not a common or ideal occlusal relationship.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the Angle's Classification and associated indices are essential in orthodontic diagnosis and treatment planning. Understanding the normal values and variations can help in identifying and correcting occlusal discrepancies.

**Correct Answer: C. 50**