450 2006 2007 300 2006 2007 150 2006 2007 0 2006 2007 2 1 (a) 12 9 6 3 0 2 1 (b) 80 60 40 20 0 2 2 (a) 4 3 2 1 0 2 2 (b) 20 THE YELLOW RIVER