A photon having 40 keV scatters from a free electron at rest. What is the maximum energy that the electron can obtain？ ( )

【單選題】A photon having 40 keV scatters from a free electron at rest. What is the maximum energy that the electron can obtain? ( )

A:7.95 eV
B:5.41 eV
C:4.32 eV
D:1.73 eV
5.41 eV