How did the skull die according to the fable Zhuangzi and the Skull?
选项：

A:Some insatiable ambition or disgraceful behavior have brought the skull to death;
B:The skull’s country had been conquered and he had been killed by his enemies.;
C:He committed suicide because he did something evil that disgraced his family;
D:How he died remained a mystery but he is happy with his life as a dead man