This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Dental
Hedstrom ﬁle having a double helix cross-section and a variation of the Uniﬁle is: 
Select and explain. 
A. Hyflex file, B. S-file, C. Unifiles, D. All of the above
B: S-file
Answer: B: S-file 
Explanation: Modification of Hedstrom ﬁle-
Hyﬂex ﬁle. It has a cross-section which presents an “s” shaped instead of the traditional single-helix  teardrop cross-section of the Hedstrom ﬁle.
Uniﬁles. They are machined from round stainless steel wire by cutting two superﬁcial grooves to  produce ﬂutes in a double-helix design. They resemble the Hedstrom ﬁle in appearance, are less  eﬃcient. They are longer marketed
S-ﬁles. It has a double-helix cross-section and is a variation of the uniﬁle.
Textbook of Endodontics Nisha GARG 3rd ED