Assume that you are a newly appointed junior auditor in PwC, an auditing firm. After a 3-month rigorous training on the job, you have been awarded the certificate of completion and is now ready to embark on your very first assignment. Today, your first job was to evaluate the audit evidence gathered by your team from Salalah Trading and see if they are reliable or not based on ISA 500. The following are the evidences gathered in various forms: a) Your senior auditor assigned you to directly observe how a sale transaction is done in Salalah Trading. You were given three days to observe the sale transaction completely. b) You need to get confirmation from customers if they really are paying their loans to Salalah Trading. The customers you have interviewed gave you their oral confirmation. c) Your colleague, the auditor in-charge of physical examination of inventory of stocks gave their written report to you as part of audit evidence. d) In the file of audit evidences, you also found the ratio analysis done by the previous auditors of Salalah Trading. e) You also collected from the Sales Department the written agreements between the company and their customers about no refund policy. They were signed by both the customers and the manager. Required: Evaluate the reliability of each of the documents above and identify the one that is considered the most reliable and least reliable.