某工程双代号网络计划如下图所示(单位:天)。该网络计划的关键线路为()。
A <img src="https://www.beizhujiaoyu.com/uploads/202208/20191207200639_2179.jpg" width="181" height="49" /> B <img src="https://www.beizhujiaoyu.com/uploads/202208/20191207200642_6338.jpg" width="516" height="46" /> C <img src="https://www.beizhujiaoyu.com/uploads/202208/20191207200646_5712.jpg" width="446" height="35" /> D <img src="https://www.beizhujiaoyu.com/uploads/202208/20191207200655_5088.jpg" width="236" height="43" />