since hard-shelled organisms evolved about 550 million years ago, billions of tons of limestone rock have been produced from their shells. limestone is made of calcium carbonate, with the formula caco3.

What are the two main ideas of this passage? museum experts feel that museums must change to attract visitors. museum experts care more about the number of visitors than about history. museums that display artworks and exhibits do not make families welcome. museums like colonial williamsburg make learning about the past fun. museums in the united states are different than in other countries.