Spiral

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Rewritten text: SOLIDWORKS 2015's primary file, sldprt, serves as the foundation for 3D designs. Explanation of changes: The original text simply listed two terms - 'SOLIDWORKS 2015' and 'sldprt'. In order to rewrite it, I first identified that 'sldprt' is a file type used in SOLIDWORKS 2015. Then, I added context by stating it as 'the primary file', which helps clarify its importance. Finally, I mentioned its function - serving as the foundation for 3D designs. The rewritten text thus provides more information and clarity without adding any external references or links.