[单选题]
A cyclotron of radiu*9/fkb *cr /cpwtsxg 2s ,mkpej gc48 0r85tt nc0.25 m accelerates protons in a 1.7-T magnetic field. What is the kinetic energy of the protons when they le8ct04ge ck,rm t8 p5jnave the cyclotron?
A. $7.8 \; keV$
B. $8.7 \; keV$
C. $7.8 \; MeV$
D. $8.7 \; MeV$
