Sonrisa has a toy car that is attached to an elastic string. Sonrisa pulls the string, applying a force to the car. Which experiment determines what happens to the car when Sonrisa pulls on the string?

a
An experiment that measures the weight of the car
b
An experiment that measures how long the string stretches
c
An experiment that measures how quickly the car speeds up
d
An experiment that measures the thickness of the string