单选题 Probably every child remembers digging a hole in his or her back yard and being told, "If you dig deep enough, you'll go to China." What would really happen if a man dug a hole through the center of the Earth and then jumped into it? The traveler entering the tunnel would first fall rapidly under the force of gravity. Eventually, as he approached the Earth's center, the jumper's weight would decrease. By the time he reached the center of the Earth, he would be weightless. An equal amount of the Earth's mass on all sides of him would cancel out the forces of gravity. Still, the traveler's original momentum would carry him past the center toward the opening on the far side of the world. After almost reaching that point, he would fall back up the hole toward his starting point. Back and forth he would then go, like a yo-yo, gradually slowing down until coming to a stop at the very center of the Earth小题:The transition that would best fit the blank space in sentence 7 is