Given the question: Information:  - Bjørvika is a neighborhood in the Sentrum borough of Oslo, Norway. The area is an inlet in the inner Oslofjord, situated between Gamlebyen and Akershus Fortress. It serves as an outlet for the river Akerselva. Since the 2000s, it has been undergoing urban redevelopment, being transformed from a container port. When completed, the Bjørvika neighborhood will be a new cultural and urban center in Oslo. The multi-purpose medium-rises of the Barcode Project is nearing its completion to the north; to the east the residential area of Sørenga is under construction. The National Opera is located at Bjørvika, and both the Oslo Public Library and the Munch/Stenersen museum is currently under construction here, the latter replacing the existing Munch Museum in 2020.   - Akerselva or Akerselven ( English : Aker River ) is a river which flows through Oslo . It starts at Maridalsvannet in Oslomarka , and follows the urban areas Nordre Aker , Sagene , Grünerløkka , Oslo centre and Grønland , whereby it finally ends at Paulsenkaien and Oset in Bjørvika . The river is considered to be a part of the Nordmarkvassdraget , and has the watercourse number 006.Z . The entire river is about 8.2 kilometres ( 5.1 mi ) long , and has a difference in elevation between source and mouth of approximately 149 metres ( 489 ft ) . Earlier , the river gave power to numerous industry companies in Oslo . Akerselven is `` Oslo 's green lung '' ; many parks and nature trails are to be found by its path , from Grønland to Maridalsvannet . A walk from along Akerselven from the rural Frysja down through the different parts of Oslo all the way down to the city centre is an amazing experience and a walk through the history of Oslo . Salmon run and spawn in the upper part of the river . Aker River in the 1970s was heavily contaminated after 150 years of industrial and sewage discharges along river . In the 1980s , a local initiative to limit emissions and revive the flora and fauna of the river began . Early in March 2011 , Oslo 's water and wastewater department accidentally discharged 6,000 litres ( 1,600 US gal ) of chlorine into the river near its headwaters , rendering the river virtually dead as of 11 March 2011 . According to Norwegian Broadcasting Corporation , the discharge occurred at the water treatment plant Oset .  - The Oslofjord is an inlet in the south-east of Norway, stretching from an imaginary line between the Torbjørnskjær and Færder lighthouses and down to Langesund in the south to Oslo in the north. It is part of the Skagerrak strait, connecting the North Sea and the Kattegat sea area, which leads to the Baltic Sea. The Oslofjord is not a fjord in the geological sense - in Norwegian the term "fjord" can refer to a wide range of waterways. The bay is divided into the inner ("indre") and outer ("ytre") Oslofjord at the point of the Drøbak Sound.    After reading the paragraphs above, we are interested in knowing the entity with which 'akerselva' exhibits the relationship of 'mouth of the watercourse'. Find the answer from the choices below.  Choices: - baltic sea  - sea  - skagerrak
The answer is:
skagerrak