Calculate the following: $(1)$ Item fill rate, $(2)$ Line fill rate, $(3)$ Order fill rate, and $(4)$ Perfect order index.

b$.$ Assume that the order was delivered on time, in the right condition, and with the proper documentation, what do you think is the perfect order fill rate? Explain your answer.

c$.$ Which metrics are considered internal and which ones are considered external? Explain your answer.