A map of the Byzantine Empire. The extent of the empire in 527 (including Turkey, the Levant, Egypt and Greece) is shaded one color. Justinian’s reconquests (North Africa, Southern Spain, Italy) are shaded another color. South of Italy is the Mediterranean Sea. North of Italy is a label for the Alps. Which of the following geographic features most likely hindered the Byzantines from expanding north of Italy?

A major river
A sea
Mountainous terrain
Desert terrain