Answer:
A) civil war
D) system of taxation
Step-by-step explanation:
The Roman army were considerably weakened as a result of the numerous Civil wars they fought which caused the loss of good soldiers and forced them to conscript men that were of lower quality, thus making the quality of the army lower.
The deaths caused by wars, famine and other factors reduced the population and thus, there were fewer people to tax, even as the economy became worse.