Detailed Instructions: In this task, you are given a context, a subject, a relation, and many options. Based on the context, from the options select the object entity that has the given relation with the subject. Answer with text (not indexes).
Q: Context: The Poltva River ( Ukrainian :  ; Polish : Petew ) is a river in the western Ukrainian Oblast of Lviv and a tributary of the Bug River . The capital of the Lviv Oblast , Lviv , is located on the river . In the 19th century the river was included into the underground sewer system of Lviv ., The Second Polish Republic, also known as the Second Commonwealth of Poland or interwar Poland, refers to the country of Poland between the First and Second World Wars (19181939). Officially known as the Republic of Poland or the Commonwealth of Poland, the Polish state was recreated in 1918, in the aftermath of World War I. When, after several regional conflicts, the borders of the state were fixed in 1922, Poland's neighbours were Czechoslovakia, Germany, the Free City of Danzig, Lithuania, Latvia, Romania and the Soviet Union. It had access to the Baltic Sea via a short strip of coastline either side of the city of Gdynia. Between March and August 1939, Poland also shared a border with the then-Hungarian governorate of Subcarpathia. Despite internal and external pressures, it continued to exist until 1939, when Poland was invaded by Nazi Germany, the Soviet Union and the Slovak Republic, marking the beginning of World War II in Europe. The Second Republic was significantly different in territory to the current Polish state. It included substantially more territory in the east and less in the west., The Bug River (or Western Bug "Zakhidnyy Buh, ") is a major European river which flows through three countries with a total length of ., Lviv (',   ', ; Yiddish: ; Latin: "Leopolis"), the largest city in western Ukraine and the seventh largest city in the country overall, is one of the main cultural centres of Ukraine. Named in honor of the Leo, the eldest son of Rus' King Daniel of Galicia. It was the capital of the Kingdom of GaliciaVolhynia (also called Kingdom of Rus') from 1272 to 1349, when it was conquered by King Casimir III the Great who then became known as the King of Poland and Rus'. From 1434, it was the regional capital of the Ruthenian Voivodeship in the Kingdom of Poland and was known as Lwów. In 1772, after the First partition of Poland, the city became the capital of the Habsburg Kingdom of Galicia and Lodomeria and was renamed to Lemberg. In 1918 in a short time was the capital of the West Ukrainian People's Republic. Between the wars, the city was known again as Lwów and was the centre of the Lwów Voivodeship in the Second Polish Republic. After World War II, it became part of the Soviet Union (Ukrainian SSR) and in 1991 of independent Ukraine. 
Administratively, Lviv serves as the administrative center of Lviv Oblast and has the status of city of oblast significance. Its population is ., Ukraine (tr. ) is a sovereign state in Eastern Europe, bordered by Russia to the east and northeast, Belarus to the northwest, Poland and Slovakia to the west, Hungary, Romania, and Moldova to the southwest, and the Black Sea and Sea of Azov to the south and southeast, respectively. Ukraine is currently in territorial dispute with Russia over the Crimean Peninsula which Russia annexed in 2014 but which Ukraine and most of the international community recognise as Ukrainian. Including Crimea, Ukraine has an area of , making it the largest country entirely within Europe and the 46th largest country in the world. It has a population of about 44.5 million, making it the 32nd most populous country in the world., Subject: poltva river, Relation: mouth_of_the_watercourse, Options: (A) main (B) sea (C) sea of azov (D) western bug
A:
western bug