What does natural selection act on?

The circles, or orbits, for electrons are called energy levels. each level can hold only a certain number of electrons. add electrons to each level until you can't add any more. how many electrons can each level hold?

Name the organs of humanelemintary canal.

Which of these is most likely to happen if parallax measurements of star distances are taken after a gap of six months? the results would be accurate the results would be inconsistent the results would not be valid the results would not be repeated