the Big Bang Theory the Conservation of Energy the Periodicity of Element Properties the Acceleration of Objects due to Gravity
I gave my students this multiple choice question: Which event or principle cannot have its validity directly tested by laboratory experiment?

Only one answer is correct among these 4 choices:
- the Big Bang Theory
- the Conservation of Energy
- the Periodicity of Element Properties
- the Acceleration of Objects due to Gravity

Could you tell me which one is correct?

the Big Bang Theory