B

Step-back instrumentation is described in three sequential phases:  
1) negotiate the canal and determine working length,  
2) enlarge and shape the apical third to a master apical file, and  
3) create coronal flare by using successively larger files each withdrawn 1 mm (“step-back”). These three steps together constitute the classic step-back technique [Walton, 1992, PMID 1572501].