By $$efault in ROS$2,$ how are Callbacks stored and processed in the callback queue?

FIFO $($First$-$In First$-$Out$)$

LRO $($Lowest Responsible Owner$)$

LIFO $($Last$-$In$,$ First$-$Out$)$

HIFO $($Highest$-$In$,$ First$-$Out$)$