I may be bombarding questions today and for that i'm sorry.  I just want to understand this chapter enough to pass.  Here is my question

To gather audit evidence concerning the proper credit approval of sales the auditor would select a sample of transaction documents from the population represened by the
a. bill of laden
b. customer order file
c. Subsidiary customer's account ledger
d. Sales invoice file

I answered b and i guess i'm getting lucky because its the right answer but i hard a hard time because a and d both make sense.  This is why they make sense. 

In order for an item to be shipped to prepare a "bill of laden" the item must have been approved to get this far. Same example for the invoices.  I guess that the customer order file that has been approved actually has the "approved" stamp and this is why it was the answer?

I still don't understand why C is correct. How does "Subsidiary customer's account ledger" show  "the proper credit approval of sales"?

I'm sorry.  I re-read my post and have updated what i really meant to say as per the choices.  Please re-read the post above...

The customer order file will have a stamp on it that states approved...