It Is Possible to Hear an Approaching Train Before You

Multiple Choice

It is possible to hear an approaching train before you can see it by listening to the sound wave through the track. If the elastic modulus is 2.0 × 10¹¹ N/m² and the density of steel is 7.8 × 10³ kg/m³, approximately how many times faster is the speed of sound in the track than in air? (v_air ≈ 340 m/s.)

A) 20
B) 5
C) 10
D) 15
E) 25