Which of the Following Was Not Cited as a Possible

Which of the following was not cited as a possible partial explanation for the decline of the Roman Empire?

A) pressure from Germanic tribesman along the Empire's northern border
B) declining agricultural production near the capital at Rome
C) the difficulties involved in administrating such a large geographic area
D) epidemic malaria in and around Rome
E) incursions into the empire by Arabic tribesmen from North Africa