A 100 gram sample of a mixture was known to contain calcium carbonate and something else, if the sample was reported to contain 45% calcium which of the following could be the other substance in the mixture?

A) NaCl

B) CaO

C) CaI2

D) Na2CO3

I know the answer is B but I don’t know how they got that answer. I need someone to show me how they work it out.