According to the theory of relativity, which of the following al-ways remains constant?

Thetheoryofrelativity,orsimply relativity,generallyencompassestwotheoriesof Albert Einstein: special relativity and general relativity.Conceptsintroducedbythetheoriesof relativity include: Measurements of various quantities are relative to the velocities of observers. In particular, space and time can dilate. Space time: space and time should be consideredtogetherandinrelationtoeachother. The speed of light is nonetheless invariant, the same for all observers.