[单选题]
A cyclotron of radius 0.2bfr,zt1pg *gzkq, naq3;6h)l 5 m accelerates protons in a 1.7-T mus9sj2g +mo.y agnetic field. What is the kinetic energy s o my.uj9+s2gof the protons when they leave the cyclotron?
A. $7.8 \; keV$
B. $8.7 \; keV$
C. $7.8 \; MeV$
D. $8.7 \; MeV$
