Teacher:You are given a science question (easy-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: A student is conducting an investigation on static electricity. The student has three balloons and rubs two of them on a piece of wool. The student then touches each balloon to a wall to observe if static electricity would cause the balloon to stick. What is the independent variable in this investigation?
(A) the height of the wall (B) the number of balloons (C) the attraction to the wall (D) the rubbing action on the wool
Student:
D