What do graphs to see?
a. the relationship between the independent variable and the dependent variable/s
b. the relationship between different dependent variables
c. the relationship between different independent variables
d. the change in data over time

show the wavelengths of a wave. show the crests of a wave.

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