Please choose the correct answer to the question: 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?

Answer1: The density of helium is less than the density of air. Answer2: The air resistance lifts the balloon up. Answer3: There is no gravity acting on the helium balloons. Answer4: The wind blows the balloon upward.

Answer format: answer1/answer2/answer3/answer4
Answer: answer1