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

A photon having 40 keV scatters from a free electron at rest.What is the maximum energy that the electron can obtain？（）
A.1.73 eV
B.5.41 eV
C.7.95 eV
D.4.32 eV
正确答案：5.41 eV