When could a dictator be appointed in Rome?

Explanation:

dictators were frequently appointed from the earliest period of the Republic down to the Second Punic War (218–201 BC), but the magistracy then went into abeyance for over a century, until it was revived in a significantly modified form, first by Sulla between 82 and 79 BC, and then by Julius Caesar between 49 and 44 ...

A) The high cost of defending the empire

Explanation:

At its height Roman empire stretched from middle est to Atlantic ocean. It was not easy to govern such a vast territory and due to its vastness the empire faced logistical and administrative issues.

Although it had excellent road systems, the Romans were still unable to effectively manage it territories. It had to struggle to maintain enough resources and troops to defend itself from rebellions and outside attacks. Emperor Hadrian even constructed wall in Britain to keep the enemies away.

As more money was spent on the military maintain the empire it affected the technological advancement. Excess expenditure on the army was one of the many reasons for the downfall of Roman empire.