单项选择题

A tunnel built in a castle is______.

A.a secret passage for the king and, queen to get out of the castle to live an ordinary life
B.a secret passage for soldiers to attack the enemy from their back
C.a secret passage for the king and queen to swim in the moat and play the chase-and-hide game in the forest
D.a secret passage for the king and queen to escape when the castle was captured