A Golfer Wants a New Set of Clubs to Improve

A golfer wants a new set of clubs to improve her game.In an experiment,she goes to the driving range and compares the distance she hits a bucket of golf balls with drivers of different materials in the head before making her purchase decision.In this simple experiment,the standardized variable is the

A) angle the ball was hit by the head surface of the club.
B) all of these except the type of material used to make the club.
C) type of material used to make the golf ball.
D) wind direction when the experiment took place.
E) force generated by her swing, when hitting the ball.