In the framework of functional testing from algebraic specifications, the strategy of test selection which has been widely and efficiently applied is based on axiom unfolding. In this paper, we propose to extend this selection strategy to a modal formalism used to specify dynamic and reactive systems. Such a work is then a first step to tackle testing of such systems more abstractly than most of the works dealing with what is called conformance testing. We get a higher level of abstraction since our specifications account for what is usually called underspecification, i.e. they do not denote a unique model but a class of models. Hence, the testing process can be applied at every design level. Keywords. Specification-based testing, dynamic specifications, selection criteria, unfolding, proof tree normalisation, exhaustivity, coalgebras.