利用箭线表示工作而在节点处将工作连接起来表示依赖关系的一种绘制项目网络图的方法是__

     The yearly Marathon in my town usually happened during a heat wave. My job was to follow behind the
runners in an ambulance   1  any of them needed medical attention.
     "We' re supposed to stay behind the  2   runner, so take it slowly," I said to the driver, Doug, as the race
started. The front-runners started to   3   and then my eyes were   4   to the woman in blue silk running shorts
and a loose white T-shirt.
     We knew we were already watching our"last runner". Her   5   were so crippled (残疾的) that it seemed
almost impossible for her to be able to walk,  6   alone run a marathon.
     Doug and I  7   in silence as she slowly moved forward.  8  , she was the only runner left in sight. Tears
streamed down my face when I watched with respect   9   she pushed forward with great  10   through the
last miles.
     When the finish line came into sight, rubbish lay everywhere and the   11   crowds had long gone home.
   12   ,standing straight and ever so proud   13   a lone man. He was  14   one end of a ribbon (缎带) of crepe
paper (彩色皱纹纸)   15   to a post. She slowly crossed through, leaving both ends of the paper fluttering
behind her.
     I do not know this woman's name, but that day she became part of my   16  -a part I often depend on. For
her, it wasn't about   17   the other runners or winning a prize, but about  18  what she had set out to do, no
matter   19   .When I think things are too difficult or i get those" I-just-can't-do-it", I think of the last runner.
Then I realize how   20   the task before me really is.