**Core Concept**
The nasolacrimal duct is a critical structure in the lacrimal apparatus, responsible for draining tears from the eye into the nasal cavity. Its length is an important anatomical feature that affects the functioning of the tear drainage system.

**Why the Correct Answer is Right**
The nasolacrimal duct is approximately 12-15 mm in length, extending from the lacrimal sac to the inferior meatus of the nasal cavity. This unique anatomical arrangement allows tears to flow from the eye into the nasal cavity, where they can be either reabsorbed or expelled. The length of the nasolacrimal duct is crucial for the proper functioning of the tear drainage system, as it enables the efficient removal of tears from the eye.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is likely incorrect, as the length of the nasolacrimal duct is not typically measured in millimeters. The correct unit of measurement is typically given in centimeters or millimeters, but the actual length is around 12-15 mm.
* **Option B:** This option is also incorrect, as the nasolacrimal duct is not typically described as being 20 mm in length. This value is significantly longer than the actual length of the nasolacrimal duct.
* **Option C:** This option is incorrect, as the nasolacrimal duct is not typically described as being 5 mm in length. This value is significantly shorter than the actual length of the nasolacrimal duct.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the nasolacrimal duct is a critical structure in the lacrimal apparatus, and its length affects the functioning of the tear drainage system. Anatomical variations in the length of the nasolacrimal duct can lead to issues with tear drainage, such as epiphora (excessive tearing).

**Correct Answer: C. 5 mm**