Time evolution of the Lorentz factor and characteristic radii, for the time-resolved spectra of GRB 160625B. Vertical gray dashed lines indicate the time intervals adopted for the spectral analysis. Panel A displays the count-rate light curve in the full NaI energy range, binned at 0.1 s. Panel B shows the Lorentz factors measured assuming that the high-energy cutoff at E_cutoff is either due to gamma-gamma opacity to pair production, or a natural break of the emission mechanisms (green and red symbols, respectively). Panel C presents the evolution of the radius at which the MeV photons are produced, and the photospheric radius (yellow and orange symbols, respectively). Arrows indicate upper or lower limits.