I'm pretty sure it's D, because I had something like this last week. but I'm sorry if it's wrong..
Valence electrons are the electron on the outer energy level and they are important because they tell if that element is compatable
OK. First, there's the heart. Then there's the blood vessels. The arteries. Then the veins. And then there's the capillaries. <span />
Hormones and water balance are another two conditions which are maintained by cells.
