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

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