The pulse of the heart causes blood circulation. For the same person, the faster the heart rate, the greater the amount of blood that enters the circulation per unit time. The red blood cells in the blood transport oxygen. Generally speaking, a person's oxygen obtained through the blood circulation per unit time The more he has, the better his physical fitness and performance. Therefore, in order to improve the athlete's level of competition in sports, they should strengthen their training in high altitude areas, because in high altitude areas, the blood content The number of red blood cells is higher than in low altitude areas. Which of the following is a hypothesis that must be assumed? ? ["The altitude does not affect the athlete's heart rate.", "Athlete's heart rate in high altitude area is not lower than in low altitude area.", "Athlete's heart rate is slower than ordinary people.", "Training at high altitudes can make athletes' heart rate faster."]