Task: Question Answering
Input (English): A balloon filled with helium gas is set free and starts to move upward. Which of the following best explains why the helium balloon moves upward?  
 (A) The density of helium is less than the density of air. (B) The air resistance lifts the balloon up. (C) There is no gravity acting on the helium balloons. (D) The wind blows the balloon upward.
Output (English): A