One of these entities named Ladoga in Russia served as a supply line to a nearby city during a year-long siege in World War II. Another, named Peipus, was the site of a 1242 battle by legendary Russian leader Alexander Nevsky. In Russia, one named Baikal is regularly passable by pedestrians and snowmobiles during winter months despite its extreme depth. For 10 points, name this occurrence, common in winter months, when a body of water becomes passable on foot.
a) Iceberg
b) Ice sheet
c) Ice floe
d) Ice road