(a) Where an application for review is filed by a permittee, OSM as well as any other person granted leave to intervene pursuant to Sec. 4.1110 shall file an answer within 20 days of service of a copy of such application.
(b) Where an application for review is filed by a person other than a permittee, the following shall file an answer within 20 days of service of a copy of such application--
(1) OSM;
(2) The permittee; or
(3) Any other person granted leave to intervene pursuant to Sec. 4.1110.