O'Connor, Alvarez, and Robbins Reply.
The preceding Comment by Xu et al. (Phys. Rev. Lett. 122, 059803 (2019); arXiv:1808.05390) erroneously applies the entropic stress expression in our Letter (T.C. O'Connor et al., Phys. Rev. Lett. 121, 047801 (2018); arXiv:1806.09509) to transient stress. In addition, the authors only apply this expression at extreme extension rates where we clearly showed deviations from the entropic stress expression for steady-state extensional flow. Hence the surprisingly minor discrepancies noted in the Comment between observed and "predicted" stress are entirely expected and have no bearing on the discussion or conclusions in our Letter.