[单选题]
A cyclotron of radius 0.25 m accelerates pro 7)v8kr6rz.z cig7t r sp4/ugwtons in a 1.7-T magnetic field. Whzxuk 62r.l+o cat is the kinetic energy of the protons wo2u+cr6 z .lxkhen they leave the cyclotron?
A. $7.8 \; keV$
B. $8.7 \; keV$
C. $7.8 \; MeV$
D. $8.7 \; MeV$
