Which of the following statements is an example of a historical argument?

a. how many people lived in the roman empire at the height of its power?
b. writing about roman history is a time-consuming and draining process for historians.
c. after the assassination of julius caesar, augustus caesar won a civil war and became roman emperor.
d. rome collapsed because of internal weaknesses, not because of attacks from foreign invaders.

I believe the answer is either B or D. I'm leaning more towards D though.
A is a question, not an argument.
B is an argument, but about writing, not much about history.
C is a fact, not an argument.
D is most likely an argument, but because I don't know much about the topic of Rome, I cannot say for sure whether this is the answer or not. I'm pretty confident that this is the correct answer though, based on the fact that this statement is not backed up with facts, while C is.

Hope this helps.