狗经过特殊训练，能跟随狐狸留下的痕迹。

The dogs are specially trained to follow the ___trail left by the fox.

他在身后留下了一串泥泞的脚印。

He left a ___trail of muddy footprints behind him.

Azman 和我看着一串新鲜的血迹通向树林。

Azman and I were looking at a ___trail of fresh blood leading into the woods.