c. The empire had grown too large to control effectively.
Explanation:
During the 4th century CE, Constantine the Great chose the capital of Byzantium as the new imperial capital. This marked a change in cultural and political influence in Rome. However, the Roman Empire continued to be unified until the government of Theodosius I. After Theodosius I, the empire proved too large to be controlled effectively, and the two emperor system became commonplace. As Rome continued to decline in importance, German invasions became more common, marking the beginning of the end for the Western half of the Empire.
