The bone building cells, once active, produce osteoid which is predominantly made up of the protein

The cell that is being described in the sentence above is the osteoblast as this is the bone building cell that is responsible of making up the bone that maintains its balance and form in the bones as it is present and important.

"it built up in the atmosphere"

i think thats it, idk, i tried looking for this exact q& a for grad. and idk if i got this question right or sorry if im wrong