Question

what do arrows c and d point to?
histology slide with arrow a pointing to small cavities. arrow b is pointing to old structures located between densely packed subunits. arrow c is pointing to rings of a structure and arrow d is pointing to a circular structure.
concentric lamellae; central canal
osteocytes; interstitial lamellae
central canal; interstitial lamellae
osteocytes; concentric lamellae

Explanation:

In bone histology, concentric lamellae are the rings - like structures that form the osteons, and the central canal is the circular structure at the center of an osteon. Arrow C points to the concentric lamellae (the rings of the osteon structure) and arrow D points to the central canal.

Answer:

concentric lamellae; central canal