#ABCR , 4 bytes

5cQx

Explanation:

5 while(peek(B)){ //Queue B peeks default to 1, so this infinite-loops
 c dequeue(c) // Queue C dequeues default to an input character if C is empty
 Q print(c) /* Prints peek at the queue, and peeks on an empty queue C
 default to the register value, which was set by `c` */
 x }

ABCR , 4 bytes

5cQx

Explanation:

5 while(peek(B)){ //Queue B peeks default to 1, so this infinite-loops
 c dequeue(c) // Queue C dequeues default to an input character if C is empty
 Q print(c) /* Prints peek at the queue, and peeks on an empty queue C
 default to the register value, which was set by `c` */
 x }