Question: Chikoy River is a tributary of which major river that flows into Lake Baikal?

Context paragraphs:
Jom-Bolok volcanic field: Jom-Bolok, also known as East Sayan Volcanic Field, is a volcanic field in Russia, 200 km west of Lake Baikal.  It is part of the Baikal rift zone which is also responsible for volcanism elsewhere around Lake Baikal.  The volcanic activity has generated long lava flows and cinder cones.  One of the lava flows is 70 km long and has a volume of 7.9 km3 .

Kirenga River: Kirenga River (Russian: Киренга ) is a river in Irkutsk Oblast in Russia, a right tributary of the Lena which flows north between the upper Lena and Lake Baikal.  The length of the river is 746 km .  The area of its basin is 46600 km2 .  The Kirenga River freezes up in late October to early November and stays under the ice until late April to May.  Its main tributaries are the Ulkan, Minya, and Khanda.  The Kirenga River begins in the Baikal Mountains 6 km west of Lake Baikal, a few dozen kilometres north of the source of the Lena.  The Baikal Amur Mainline follows and crosses the Kirenga between Magistralnyy and Ulkan.  It joins the Lena at the town of Kirensk.

Pribaikalsky National Park: Pribaikalsky National Park (also spelled Pribaykalski, Russian: Прибайкальский национальный парк ) covers the southwest coast of Lake Baikal in southeastern Siberia.  The coastal strip includes some mountain ridges to the west as well as offshore islands such as Olkhon Island to the east.  It is about 50 km southeast of the city of Irkutsk, Irkutsk Oblast.  The park is managed with three other nature reserves, and is a major component of the UNESCO World Heritage Site "Lake Baikal".  The Angara River, which is the outflow of Lake Baikal west into the Yenisei River basin, runs through the park.  The park has very high levels of biodiversity and endemic species.

Dzherginsky Nature Reserve: Dzherginsky Nature Reserve (Russian: Джергинский заповедник ) is a Russian 'zapovednik' (strict nature reserve), located about 100 km east of the northern section of Lake Baikal.  It covers the source and upper reaches of the Barguzin River, the second largest tributary to Lake Baikal, and is at the junction of three mountain ranges - the Barguzin Range to the west of the reserve, the Ikat Range and the South Muya Ridge.  The reserve's mountainous territory is dominated by larch forests.  It is situated in the Kurumkansky District of Buryatia.  The nearest city, Ulan-Ude, is 560 km to the south.  The reserve was formally established in 1992 to protect the biodiversity of the upper Barguzin valley, and to study natural processes of the area.  It covers an area of 238088 ha .

Baikal-Lena Nature Reserve: Baikal-Lena Nature Reserve (Russian: Байкало-Ленский заповедник ) (also Lake Baikal; or Baykal-Lensky, or Baykal-Lena) is a Russian 'zapovednik' (strict nature reserve) located on the northwest coast of Lake Baikal in southern Siberia.  It protects both lake shore and the source of the Lena River.  The reserve stretches along the western coast of Lake Baikal for about 120 km, with an average width of 65 km.  The reserve is situated in the Kachugsky District of Irkutsk Oblast.  As of December 1996, the Baikal-Lena Reserve (combined with the Barguzin and Baikal Reserves) is included in the list of sites of World Cultural and Natural Heritage, and a World Heritage Site with Pribaikalsky National Park.  The Baikal-Lena Reserve is managed jointly with Pribaikalsky National Park, which is immediately to the south, thus protecting a continuous 580 km of shoreline on Lake Baikal.

Selenga River: The Selenga River (Selenge River, Mongolian: Сэлэнгэ мөрөн, "Selenge mörön"; Buryat: Сэлэнгэ гол, "Selenge gol", Сэлэнгэ мүрэн, "Selenge müren" ; Russian: Селенга́ ) is a major river in Mongolia and Buryatia, Russia.  Its source rivers are the Ider River and the Delgermörön river.  It flows into Lake Baikal and has a length of 992 km or 1024 km , according to other sources.  The Selenga River is the headwaters of the Yenisei-Angara River system.  Carrying 935 m3/s of water into Lake Baikal, it comprises almost half of the riverine inflow and forms a wide delta of 680 km2 , when it reaches the lake.

Vitim River: Vitim River (Russian and Buryat: Витим "Vitim" ; Evenki: Витым "Witym" ; Yakut: Виитим "Viitim" ) is a major tributary of the Lena River.  With its source east of Lake Baikal, the Vitim flows 1978 km north through the Transbaikal Mountains and the town of Bodaybo.  The river peaks in June and freezes from November to May.  It is navigable from the Lena to Bodaybo.  Upstream, tugs can haul barges as far as the Baikal Amur Mainline (BAM), but this is becoming rare.  Formerly, because of its swift current, goods were hauled 144 km overland from Chita to a place called Romanovka.  There boats were built, floated down the river, and broken up at their destination.  This lasted until the late 1940s.  The Vitim is an excellent place for adventure rafting, but is rarely visited because of its isolation.

Chikoy River: Chikoy (Russian: Чикой , Mongolian: Цѳх гол , Buryat: Сүхэ гол ) is a river in Zabaykalsky Krai and the Buryat Republic in Russia, which partially flows along the Russo-Mongolian border.  It is a right tributary of the Selenga River.  The length of the Chikoy is 769 km .  The area of its basin is 46200 km2 .  The river usually freezes over in late October or early November and stays icebound until April or early May.  Its largest tributary is the Menza River.

Upper Angara River: The Upper Angara River (Russian: Верхняя Ангара , "Verkhnyaya Angara"; Buryat: Дээдэ Ангар , "Deede Angar") is a river in Siberia to the north of Lake Baikal.  It is 320 km long and rises north-east of Lake Baikal, flowing south-west through the Buryat Republic and eventually into the lake.  It is partly navigable.  The Baikal Amur Mainline runs along the north side of the river northeast up its valley, crossing between Anamakit and Novy Uoyan and crossing the river a second time upstream before heading into the mountains.

Chikoy National Park: Chikoy National Park (Russian: Чикой, национальный парк ) was officially created in 2014, on the mountain-steppe border region of south-central Siberia and Mongolia.  It is in the Krasnochikoysky District of the Zabaykalsky ("Trans-Baikal") administrative region of Russia.  This places the park about 250 miles southeast of Lake Baikal, on the edge of the Lake Baikal ecoregion.  The stated purpose for creating the park is the "preservation of unique natural complexes in the upper reaches of the river Chikoy".  While tourism and sport fishing will be encouraged, hunting and roads will be prohibited.

Supporting paragraphs: Chikoy River Selenga River
Answer: Selenga River