Sentence:  Two principal search strategies were (correctly) predicted: Type-A programs that apply "brute force" inspection of every possible position over a fixed number of <WSD>plys</WSD>; and Type-B programs that prune potential moves according to some selection function and then examine the significant sets over as many <WSD>plys</WSD> as practical and only at those positions reflecting a degree of stability. 
Meaning of the word marked with <WSD> tags:  In two-player sequential games, a "half-turn" or a move made by one of the players.