Task: Question Answering
Input (English): 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
Output (English): D