(a) (1) The testimony of witnesses in inter partes cases may be submitted in the form of an affidavit or a declaration pursuant to § 2.20 and in conformance with the Federal Rules of Evidence, filed during the proffering party's testimony period, subject to the right of any adverse party to elect to take and bear the expense of oral cross-examination of that witness as provided under paragraph (c) of this section if such witness is within the jurisdiction of the United States, or conduct cross-examination