What gas was present in earth's earliest atmosphere and used by cyanobacteria during photosynthesis?
a.
oxygen
b.
carbon dioxide
c.
nitrogen
d.
methane

When can a theory be modified if a new type of technology allows for new observations that raise new questions? a-immediately, while the questions about the theory are being asked b-after new hypotheses related to the theory are tested in experiments c-only after the new observations disprove all parts of the theory d-while scientists begin to think about how the theory could improve