Despite their blank structure, human bones are surprisingly blank.

A hollow; string

B solid; lightweight

C compact; flexible

D spongy; fragile

Despite their hollow structure, human bones are surprisingly strong. Human bones have a hollow cylindrical shape within them, which holds our blood (also known as marrow). Still, even though they are hollow, they are extremely durable, which is why getting hit in the arm with something doesn't always break it. We were built to endure harsh treatment, to a certain extent.