The Commission may grant a certificate B when the applicant does not propose to operate regularly upon a fixed schedule or route, but only desires to operate over a particular route or routes which are not already served by the holder of a certificate A, but will operate in instances when by his solicitation or otherwise he has procured passengers to be transported over the route or routes designated in his application. In the granting of such a certificate B the Commission may consider the public convenience and necessity and whether the territory proposed to be served is already served by a carrier. If the public convenience and necessity require the granting of more than one certificate B over such route or routes, the Commission may grant an additional certificate B, but such certificate may be revoked by the Commission at the end of any license year as to any particular route or routes, if prior to the expiration of such year the Commission has granted over such route a certificate A.