any abuse by one or more undertakings of a dominant position within the common market or in a substantial part of it shall be prohibited as incompatible with the common market in so far as it may affect trade between Member States. Such abuse may, in particular, consist in: a) directly or indirectly imposing unfair purchase or selling prices or other unfair trading conditions; b) limiting production, markets or technical development to the prejudice of consumers; c) applying dissimilar conditions to equivalent transactions with other trading parties, thereby placing them at a competitive disadvantage; d) making the conclusion of contracts subject to acceptance by the other parties of supplementary obligations which, by their nature or according to commercial usage, have no connection with the subject of such contracts.