21. A person who has restricted his availability for employment in accordance with regulation 17(1) or, as the case may be, regulation 17(2), shall in any week in which he has so restricted his availability for not less than 3 days be treated as actively seeking employment in that week if he takes such steps as he can reasonably be expected to have to take in order to have the best prospects of securing employment for which he is available under regulation 17.