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: John and Keith are neighbors. They have been pondering about how to communicate with each other in a doomsday scenario when all the electronic devices would be useless. They connected their houses with three ducts. 
One of the ducts is filled with air; they called it channel A. Another duct is filled with water; they 
called it channel B. And the last duct is filled with iron; they called it channel C. They can now transmit sound with these channels of communication; in case, disaster strikes. 
Question: Which channel would be the slowest, channel A,  channel B, or channel C?
Output (English): channel A.