Given the question: Information:  - Kozelsk is a town and the administrative center of Kozelsky District in Kaluga Oblast, Russia, located on the Zhizdra River (Oka's tributary), southwest of Kaluga, the administrative center of the oblast. Population:   - Nizhny Novgorod, colloquially shortened to Nizhny, is a city in the administrative center of Nizhny Novgorod Oblast and Volga Federal District in Russia. From 1932 to 1990, it was known as Gorky, after the writer Maxim Gorky, who was born there. The city is an important economic, transportation, scientific, educational and cultural center in Russia and the vast Volga-Vyatka economic region, and is the main center of river tourism in Russia. In the historical part of the city there are a large number of universities, theaters, museums and churches. Nizhny Novgorod is located about 400 km east of Moscow, where the Oka empties into the Volga. Population:   - Zhizdra River ( Russian :  ) is a river in Kaluga Oblast in Russia , Oka 's left tributary . The length of the river is 223 km. The area of its basin is 9,170 km² . the Zhizdra River freezes up in late November and stays icebound until early April . Its main tributaries are the Resseta , Vytebet , and Serena . The towns of Kozelsk and Zhizdra are located on the shores of the Zhizdra . The name is of Baltic origin ( cf. Lithuanian žizdras ' thick sand , gravel ' ) . The lower course of the Zhizdra is located in Ugra National Park .  - Russia (from the  Rus'), also officially known as the Russian Federation, is a country in Eurasia. At , Russia is the largest country in the world by surface area, covering more than one-eighth of the Earth's inhabited land area, and the ninth most populous, with over 140 million people at the end of March 2016. The European western part of the country is much more populated and urbanised than the eastern, about 77% of the population live in European Russia. Russia's capital Moscow is one of the largest cities in the world, other major urban centers include Saint Petersburg, Novosibirsk, Yekaterinburg, Nizhny Novgorod and Samara.  - Kaluga is a city and the administrative center of Kaluga Oblast, Russia, located on the Oka River southwest of Moscow. Population:   - Saint Petersburg is Russia's second-largest city after Moscow, with five million inhabitants in 2012, and an important Russian port on the Baltic Sea. It is politically incorporated as a federal subject (a federal city). Situated on the Neva River, at the head of the Gulf of Finland on the Baltic Sea, it was founded by Tsar Peter the Great on May . In 1914, the name was changed from Saint Petersburg to Petrograd, in 1924 to Leningrad, and in 1991 back to Saint Petersburg. Between 17131728 and 17321918, Saint Petersburg was the imperial capital of Russia. In 1918, the central government bodies moved to Moscow.  - Novosibirsk is the third-most populous city in Russia after Moscow and St. Petersburg. It is the most populous city in Asian Russia, with a population of 1,473,754 as of the 2010 Census. It is the administrative center of Novosibirsk Oblast as well as of the Siberian Federal District. The city is located in the southwestern part of Siberia on the banks of the Ob River adjacent to the Ob River Valley, near the large water reservoir formed by the dam of the Novosibirsk Hydro Power Plant, and occupies an area of .  - Europe is a continent that comprises the westernmost part of Eurasia. Europe is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, and the Mediterranean Sea to the south. To the east and southeast, Europe is generally considered as separated from Asia by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways of the Turkish Straits. Yet the non-oceanic borders of Europea concept dating back to classical antiquityare arbitrary. The primarily physiographic term "continent" as applied to Europe also incorporates cultural and political elements whose discontinuities are not always reflected by the continent's current overland boundaries.  - Eurasia is the combined continental landmass of Europe and Asia. The term is a portmanteau of its constituent continents. Located primarily in the Northern and Eastern Hemispheres, it is bordered by the Atlantic Ocean to the west, the Pacific Ocean to the east, the Arctic Ocean to the north, and by Africa, the Mediterranean Sea, and the Indian Ocean to the south. The division between Europe and Asia as two different continents is a historical and cultural construct, with no clear physical separation between them; thus, in some parts of the world, Eurasia is recognized as the largest of five or six continents. In geology, Eurasia is often considered as a single rigid megablock. However, the rigidity of Eurasia is debated based on the paleomagnet data.  - A tributary or affluent is a stream or river that flows into a larger stream or main stem (or parent) river or a lake. A tributary does not flow directly into a sea or ocean. Tributaries and the main stem river drain the surrounding drainage basin of its surface water and groundwater, leading the water out into an ocean.    Given the information above, choose from the list below the object entity that exhibits the relation 'mouth of the watercourse' with the subject 'zhizdra river'.  Choices: - arctic ocean  - atlantic ocean  - indian ocean  - main  - neva  - ob  - oka river  - pacific ocean  - sea  - volga
The answer is:
oka river