比赛时有人违例，此时裁判鸣哨并以单臂上举之手势，应怎么判罚（）
A、间接自由球
B、直接自由球
C、角球
D、12码球
参考答案：A
前序法是先访问 【4】 ，然后按前序周游左子树，按前序周游右子树。
We have been excavating layers of ash almost six metres thick, which suggests that they________the fire burning all winter.
A．should keep
B．might have kept
C．should have kept
D．might keep
