What has caused most of the mass extinctions on earth?

Which of these organisms have the greatest amount of cell specialized

In which state was more than 100 billion tons of coal mined in 2005?

The double bond between a carbon atom and two oxygen atoms (a molecule of carbon dioxide) has two characteristics. what are they? a.an ionic bond is formed between the oxygen and carbon atoms. b.four valence electrons are shared. c.two valence electrons are shared. d.valence electrons are shared between oxygen atoms.