How does a mule work

A．A person who sends smuggling goods for a syndicate is called mule.
B．A person in charge of smuggling goods is called mule.
C．A person who makes delivery for a syndicate is called mule.
D．A person who receives instructions from a smuggler is called mul

A．To show how a smuggler is caught.
B．To show a smuggler is afraid of the police.
C．To show to keep a wary eye on couriers is useless.
D．To show mules may keep the profit for themselves.