Task: Question Answering
Input (English): Passage: Bulgaria occupies a portion of the eastern Balkan peninsula, bordering five countries—Greece and Turkey to the south, Macedonia and Serbia to the west, and Romania to the north. The land borders have a total length of 1,808 kilometres (1,123 mi), and the coastline has a length of 354 kilometres (220 mi). Its total area of 110,994 square kilometres (42,855 sq mi) ranks it as the world's 105th-largest country. Bulgaria's geographic coordinates are 43° N 25° E. The most notable topographical features are the Danubian Plain, the Balkan Mountains, the Thracian Plain, and the Rhodope Mountains. The southern edge of the Danubian Plain slopes upward into the foothills of the Balkans, while the Danube defines the border with Romania. The Thracian Plain is roughly triangular, beginning southeast of Sofia and broadening as it reaches the Black Sea coast.
The Balkan mountains run laterally through the middle of the country. The mountainous southwest has two distinct alpine ranges—Rila and Pirin, which border the lower but more extensive Rhodope Mountains to the east. Musala, at 2,925 metres (9,596 ft), is the highest point in both Bulgaria and the Balkan peninsula, and the Black Sea coast is the country's lowest point. Plains occupy about one third of the territory, while plateaux and hills occupy 41%. Most rivers are short and with low water levels. The longest river located solely in Bulgarian territory, the Iskar, has a length of 368 kilometres (229 mi). Other major rivers include the Struma and the Maritsa in the south.Bulgaria has a changeable climate, which results from being positioned at the meeting point of the Mediterranean and continental air masses combined with the barrier effect of its mountains. Northern Bulgaria averages 1 °C (1.8 °F) cooler, and registers 200 millimetres (7.9 in) more precipitation, than the regions south of the Balkan mountains. Temperature amplitudes vary significantly in different areas. The lowest recorded temperature is −38.3 °C (−36.9 °F), while the highest is 45.2 °C (113.4 °F). Precipitation averages about 630 millimetres (24.8 in) per year, and varies from 500 millimetres (19.7 in) in Dobrudja to more than 2,500 millimetres (98.4 in) in the mountains. Continental air masses bring significant amounts of snowfall during winter. 
Question: What country do the Balkan mountains run laterally through?
Output (English): Bulgaria.