<image>Newton's discovery of the "Law of Universal Gravitation" is said to have originated from his childhood experience of reading under an apple tree, when a ripe apple suddenly fell and landed on his head. In the midst of the pain, he wondered: why does the apple fall down instead of "flying up"? With this question in mind, he continued learning and exploring, eventually discovering the "Law of Universal Gravitation" and other laws, becoming a world-renowned scientist. The following graph can roughly reflect the change in the velocity V of the apple with respect to time t during its fall ___ (fill in the number).