Foraminifera are predatory protista with , which is the ultimate source

Foraminifera are predatory Protista with a CaCO3 (Calcium carbonate) shell, which is the ultimate source of Limestone.

Explanation:

Forams or foraminifers are types of protists whose bodies are composed of only single cell. They have a shell or test around their body that is composed on Calcium carbonate (in one specie it is composed of silica).

After their death, the shells accumulate at the bottom of sea to give rise to large deposits of biogenic carbonates specially limestone (CaCO3). This limestone also serves to be a source of Carbon. This process is not rapid and may take millions of years to give rise to considerable rocks and deposits of limestone. This limestone is commercially used by men for a variety of purposes like manufacture of glass, steel etc.

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