Task: Question Answering
Input (English): Background Paragraph: In common everyday speech, speed of sound refers to the speed of sound waves in air. However, the speed of sound varies from substance to substance: sound travels most slowly in gases; it travels faster in liquids; and faster still in solids. For example, (as noted above), sound travels at 343 m/s in air; it travels at 1,480 m/s in water (4.3 times as fast as in air); and at 5,120 m/s in iron (about 15 times as fast as in air). In an exceptionally stiff material such as diamond, sound travels at 12,000 metres per second (27,000 mph);[1] (about 35 times as fast as in air) which is around the maximum speed that sound will travel under normal conditions. 
Story: A popular radio station was playing the classic rock songs. There was a radio in a car with a lot of obstacles between the station. There was a fire with a lot of smoke, a river, and a mountain. The man driving the car had a great time listening to the classics. 
Question: Where was the radio signal traveling the slowest: river or mountain?
Output (English): river.