Context: Lily notices that her cell phone charges more quickly sometimes and more slowly other times. She wonders what factors affect how quickly a phone charges. So, she decides to design an experiment. She has the following supplies available:
one cell phone
a two-foot-long charging cable
a five-foot-long charging cable
a stopwatch
a wall outlet
Using only these supplies, which question can Lily investigate with an experiment?
A. Does a cell phone charge faster when plugged into a wall outlet or into a computer?
B. Does a tablet charge faster with a two-foot-long charging cable or a five-foot-long charging cable?
C. Does a cell phone charge more quickly when it is turned on or when it is turned off?
From the given choices, pick the correct answer and respond by indicating the letter of the correct option.

C