68. The Auditor so appointed may,-(a) by written summons, require the production before him of any document which he may consider necessary for the proper conduct of the audit;(b) by written summons, require any person having the custody or control of, or being accountable for, any such document to appear in person before him; and(c) require any person so appearing before him to make and sign a declaration with respect to any such document to answer any questions, or to prepare and submit any statement.