The smallest piece of matter that still has the properties of an element is a(n) 1.compound 2.electron
3.atom
4.nucleus

I think its number 3, correct me if I'm wrong.

the skeletons of most mammals reach a certain size and then stop growing. however, many animals, including some mammals, keep growing throughout their lives. kangaroos, for example, just keep growing and growing until they die. also, the longer an animal lives the greater the chance it might catch some deadly disease.