Please choose the correct answer to the question: It is necessary to add a day to the calendar every four years because

Answer1: the axis of Earth is tilted. Answer2: the gravitational pull of the Sun affects Earth's revolution. Answer3: the revolution of Earth is not exactly 365 days. Answer4: the Moon crosses the orbit of the Sun every 28 days.

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