refer to figure 21-18. bundle c represents a point where
10 a. mrs, y > py/px.
o b. mrsxy= px/py
o c. mrsxy < px/py
o d. mrsxy > px/py

it's when something is going so fast it makes a loud noise, breaking the sound barrier, for example: a jet makes a loud noise, not only cause of the jet but because they reached a point where they went so fast that you can year the noise from various parts, kinda like a police car, lets say the car was going   so fast that you hear the siren of the cop car right next to you, but the car is probably a mile a way, it's cos the siren was next to you a few seconds if you understand me