Which of the following is statements is generally correct about audit evidence?

a. Appropriateness of audit evidence refers to audit evidence obtained from outside the entity

b. Appropriateness of audit evidence refers to the amount of corroborative evidence obtained

c. The more effective the internal control structure, the more assurance it provides about the reliability of the accounting data and financial statements

d. Information obtained indirectly from independent outside sources is more persuasive than the auditor's direct personal knowledge obtained through observation and inspection