How was Einstein's theory that energy and mass are interchangeable eventually proven to be correct?

Stanley Eddington confirmed the theory by his observations and inferences using the solar eclipse

Explanation:

Albert Einstein proposed a theory that energy and mass are interchangeable; termed the theory of special relativity. This can be observed when an object moves with the speed of light. The theory states that all laws in physics are the same for all non-accelerating observers.

Thus,

                      E = m

This theory could not be proven over many years, therefore it was not fully accepted.

In 1919, Stanley Eddington confirmed the theory by his observations and inferences using solar eclipse. The theory is now very accurate at any given speed when gravitational force tends to zero. This has lead to other observations to prove Einstein's theory.

i am pretty sure it is c because when people are closer together, they cannot move as freely. picture being in a mostly full elevator, for instance. the people are right next to you, and you can barely move.