Final answer:
The singularity of a black hole is considered unknown because we do not have a complete understanding or mathematical tools to describe it. Singularities are points of zero volume and infinite density where spacetime ceases to exist.
Step-by-step explanation:
The statement from Part A, 'the singularity of a black hole has infinite density,' is considered unknown because we do not yet have the physical understanding or the mathematical tools to describe the singularity itself, or even if singularities actually occur.
A singularity is a point of zero volume and infinite density where spacetime ceases to exist. The laws of physics as we know them break down at the singularity.
Although we don't have a complete understanding of singularities, from the outside, a black hole can be described as a singularity surrounded by an event horizon.