This paper presents a comprehensive spectroscopic analysis of the exceptional gamma-ray burst (GRB) 221009A, combining data from the GECAM-C and Fermi/GBM instruments. GRB 221009A is the brightest GRB ever observed, with isotropic energies reaching up to $\sim 10^{55}$ erg. The study focuses on the search for emission and absorption lines, which are crucial for understanding the physics of GRBs. Using GECAM-C, which provided accurate measurements even during the most luminous part of the burst, and Fermi/GBM, which covered higher energy bands, the authors identified prominent emission lines in multiple time intervals. The central energy of the Gaussian emission line evolved from about 37 MeV to 6 MeV, with a nearly constant ratio (about 10%) between the line width and central energy. The line energy and flux evolved with time as power-law decays with indices of -1 and -2, respectively. The authors suggest that these emission lines likely originate from blue-shifted electron-positron pair annihilation at 511 keV. They propose that the emission lines come from dense clumps with electron-positron pairs traveling with the jet, allowing for the first direct measurement of the bulk Lorentz factor of the jet ($\Gamma \sim t^{-1}$) during the prompt emission. The findings provide new insights into the physics of GRBs and relativistic jets.This paper presents a comprehensive spectroscopic analysis of the exceptional gamma-ray burst (GRB) 221009A, combining data from the GECAM-C and Fermi/GBM instruments. GRB 221009A is the brightest GRB ever observed, with isotropic energies reaching up to $\sim 10^{55}$ erg. The study focuses on the search for emission and absorption lines, which are crucial for understanding the physics of GRBs. Using GECAM-C, which provided accurate measurements even during the most luminous part of the burst, and Fermi/GBM, which covered higher energy bands, the authors identified prominent emission lines in multiple time intervals. The central energy of the Gaussian emission line evolved from about 37 MeV to 6 MeV, with a nearly constant ratio (about 10%) between the line width and central energy. The line energy and flux evolved with time as power-law decays with indices of -1 and -2, respectively. The authors suggest that these emission lines likely originate from blue-shifted electron-positron pair annihilation at 511 keV. They propose that the emission lines come from dense clumps with electron-positron pairs traveling with the jet, allowing for the first direct measurement of the bulk Lorentz factor of the jet ($\Gamma \sim t^{-1}$) during the prompt emission. The findings provide new insights into the physics of GRBs and relativistic jets.