Why do experiment need to be repeatable ?

Which of the following statements is true? a. elements in the last period are radioactive. b. atomic weight is the same as atomic mass. c. elements in the same group have the same number of electron shells. d. atomic number equals the number of neutrons in the nucleus of an atom.

Arollercoaster car at the top of a hill has potential energy kinetic energy chemical energy light energy

Which type of fossil does this image depict?