A machine in a candy factory produces 5 and 1/2 batches of milk balls every 3/4 hour. Which of the following is the unit rate for producing the milk balls, in batches per hour?

a
4 and 1/8 : 1
b
5 and 3/4 : 1
c
6 and 1/4 : 1
d
7 and 1/3 : 1