Question

During the twentieth century, the study of the large-scale structure of the universe evolved from the theoretical to the practical; the field of physical cosmology was made possible because of both ___________ the better ability to observe extremely distant astronomical objects.
(A) because of both Einstein's theory of relativity and
(B) by both Einstein's theory of relativity and
(C) by Einstein's theory of relativity and also
(D) because of Einstein's theory of relativity and also
(E) as a result of both Einstein's theory of relativity and

Answer 1

During the twentieth century, the study of the large-scale structure of the universe evolved from the theoretical to the practical; the field of physical cosmology was made possible because of both Einstein's theory of relativity and the better ability to observe extremely distant astronomical objects.  Option A is correct.

Albert Einstein formulated two theories which he included to his theory of relativity. On the one hand the special relativity and on the other hand,  general relativity.

The former refers to the lack of gravity and the physical phenomena that occurs because of it and the latest refers to the law of gravitation.