The calcium carbonate found in limestone was originally extracted from
a. seawater
b. shale
c. magma
d. silt

The calcium carbonate found in limestone was originally extracted from A. SEAWATER.

Limestone is a sedimentary rock. It was formed from small rock or stone particles that have been compacted by pressure. It can be formed either by evaporation or with the help of living organisms.

Calcium carbonate in seawater are used by ocean-dwelling organisms to create their shells and bones; like corals, clams, oysters. When these organism die, their shells and bones are broken down by the waves and settle on the ocean floor where the pressure of the ocean water drives these sediments together to create limestone.

Limestone is created through evaporation. When water containing calcium carbonate evaporates it leaves behind the sediment deposit which in turn becomes limestone after being subjected to the water pressure.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.