Context: Monica 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 Monica investigate with an experiment?
A. Does a tablet charge faster with a two-foot-long charging cable or a five-foot-long charging cable?
B. Does a cell phone charge more quickly when it is turned on or when it is turned off?
C. Does a cell phone charge faster when plugged into a wall outlet or into a computer?
Determine the correct option from the provided choices and reply with its corresponding letter.

B