Question: Four students are investigating the effect of the force of a baseball bat on a ball. They marked the four different points where each of the balls made contact on the bat after each strike by each of the four students. Which of the following describes an error in the experimental design?

  Options: 
 A. failure to conduct enough trials
 B. not considering the spin of the ball
 C. not using balls of different masses
 D. failure to define one testable variable 
Answer: D