D, The lack of a strong economy.
The Roman economy underwent inflation after the reign of emperor Marco Aurelio. Gold income from new territories to Rome began to slow down.
The Romans had also spent a ton of gold to pay for luxuries, so there was less gold to be made into coins. This caused coins to become less valuable because they contained less gold.
Because of this, merchants raised the prices of the goods they were selling, so people started to trade items instead of using coins at all.
