The transmission includes a first shaft, a second shaft, a third shaft, and a fourth shaft. These shafts are parallel to each other. A first gear couples to the first shaft. A second gear couples to or idles around the first shaft when a first clutch is switched. A third gear meshes with the first gear and couples to the second shaft. A fourth gear couples to or idles around the second shaft when a second clutch is switched. A sixth gear meshes with the first gear and couples to the third shaft. A seventh gear couples to or idles around the third shaft when a fourth clutch is switched. A ninth gear meshes with the second gear, the fourth gear, and the seventh gear, and couples to the fourth shaft.