Answer:
In other words, it is so old that it is difficult to imagine.
Explanation:
In this case, the simplest answer is the correct one. It's not about math or science; it's just what it is; the history of the universe is very hard to imagine because is humans haven't been in it for very long.
Answer:
B (the one you selected)
Explanation:
A is not right since many stories use more than one theme, especially in arcs.
C is not right since the subject is what the story is about, which can not line up with the theme.
D is not right because most real world examples do not do this.
The best answer is being combative ;)