Answer:
D. siege warfare
Step-by-step explanation:
Ardashir I centralized the government and reorganized the military according to the Achaemenid decimal system, bringing it directly under his control. He utilized both the Seleucid and Parthian body armor, kept the Parthian cavalry units, expanded his infantry (again, in line with the Achaemenid system), employed Roman tactics, and also made use of their technology of siege engines and other devices. The Sassanians, last of the ancient Persian empires, fell to the Arabs in 651 CE, and their army and navy were disbanded. In its time, however, the Sassanian army represented the best version of the Persian military, among the greatest fighting forces of the ancient world.