Which of these is not a part of the circulatory system?
1)heart
2)blood
3)arteries
4)femur

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?

Plz ! ive been stuck on this forever

Of your good in bio kusing a series of preliminary observations; pstate a problem developed from these observations, formulate a hypothesis, design an experiment to test the hypothesis