Which of the following statements is true regarding an auditor's expert?（ ）

A、The expert must be a partner or member of staff of the audit firm.
B、Provided that the auditor has assessed the expert to be competent audit evidence provided by the expert can be relied upon.
C、Unless required to do so by law or regulation the auditor must not refer to the expert in an auditor's report which contains an unmodified opinion.