Answer:
A. Baltic Sea
Explanation:
The Baltic Sea is shown below. As you can see, both Russian cities are important ports with a very long history of trade and military presence in the region.
During the Middle Ages, a very important group of states formed the Hanseatic League, that mainly benefited from the Baltic Sea and fostered trade.
The importance of the Sea is that it connects the Baltic states, the northern cities of Luebeck, Danzig, Riga, Novgorod.
The Russian enclaves of St. Petersburg and Kaliningrad serve as an area of influence to the northeast of Europe.
Together they both contribute to a bast area where important routes of trade take place and to the present, this area is very active economically. Some may also consider the area a source of tension , as the former Baltic states tend to commerce with the EU , formerly they served Moscow interests. The presence of Kaliningrad still enables today some maneuvers by Moscow.
