Correct Option: C

The BLAST program sets a size k for k-tuple subwords. The program then looks for diagonals in the comparison matrix between query and search sequence along which many k-tuples match. This program also aims at identifying core similarities for later extension. The core similarity is defined by a window with a certain match density on DNA or with an amino acid similarity score above some threshold for proteins. Independent of the exact definition of the core similarity, BLAST rests on the precomputation of all strings which are in the given sense similar to any position in the query. n BLAST nomenclature this set of strings is called the neighborhood of the query. The code to generate this neighborhood is in fact exceedingly fast.