Each element has its own emission and absorption lines. What is the best explanation for this? Each element has a specific number of protons and neutrons.
Each element has a distinct number of electrons.
Each element has a unique set of physical characteristics.
Each element has unique energy levels.

Which of the following situations is impossible? a) an object has velocity directed east and acceleration directed east. b) an object has zero velocity but non-zero acceleration. c) an object has constant non-zero velocity and changing acceleration. d) an object has velocity directed east and acceleration directed west. e) an object has constant non-zero acceleration and changing velocity.