Marty has to audit the sales transactions of Okawa Inc. Okawa supplies tools to the mining industry and carries a large number of different makes and models of standard mining tools. Okawa also designs and manufactures tools for special purposes and for miners operating in difficult conditions. The custom-designed tools are made only on the signing of a contract and receipt of a deposit, whereas standard tools are supplied to regular customers on receipt of a phone or email order. Okawa's sales transactions vary from a few dollars to millions of dollars depending on the number of items sold, whether the individual items are large or small tools, and whether the tools are standard items or custom designed. Marty is instructed to gather evidence about the sales transactions using sampling and vouching. This is explained in detail in the audit program. Required a. Explain how Marty would select a sample of sales transactions as well as vouch the sales transactions. What primary assertion is Marty testing with the vouching procedure? b. How could Marty use ADA as a substantive procedure?