设G是有n个结点，m条边的连通图，必须删去G的(    )条边，才能确定G的一棵生成树．

<img src、"http://oss.ouchn.cn/ddzx/N601/jiaoyu/lssx/images/xk/3/image012.png" alt="">

<img src、"http://oss.ouchn.cn/ddzx/N601/jiaoyu/lssx/images/xk/3/image013.png" alt="">

<img src、"http://oss.ouchn.cn/ddzx/N601/jiaoyu/lssx/images/xk/3/image014.png" alt="">

<img src、"http://oss.ouchn.cn/ddzx/N601/jiaoyu/lssx/images/xk/3/image015.png" alt="">