The following table lists the speed of sound in various materials. Use this table to answer the question. Substance Speed (m/s)
Glass 5,200
Aluminum 5,100
Iron 4,500
Copper 3,500
Salt water 1,530
Fresh water 1,500
Mercury 1,400
Hydrogen at 0°C 1,284
Ethyl Alcohol 1,125
Helium at 0°C 965
Air at 100°C 387
Air at 0°C 331
Oxygen at 0°C 316

How long would it take for a sound impulse to travel through a copper rod 25 kilometers long?

0.071 sec
7.1 sec
140 sec
0.14 sec