There is a layer of cells indicated by the label "A".  What type of cells are these?  

This cell sits between the ependymal and endothelial cells, similar to the blood-brain barrier where they also sit between astrocytes and endothelial cells

What is found in the region indicated by label "B"?

The tissue structure around label "C" responsible for CSF production is what?

What is found in the region indicated by the label "D"?

Cilia are present on the surface of what cells?