Teacher:You are given a science question (hard-level) and four answer options (associated with "A", "B", "C", "D"). Your task is to find the correct answer based on scientific facts, knowledge, and reasoning. Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D'. There is only one correct answer for each question.
Teacher: Now, understand the problem? Solve this instance: 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?
(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
Student:
D