[单选题]
A cyclotron of radius 0.25 m accec7b mw5z y)kkppru 6)5lerates protons in a ixenh 40* u9ye1.7-T magnetic field. What is the kinetic energy of the protons when they leave thehue y*4ixn 09e cyclotron?
A. $7.8 \; keV$
B. $8.7 \; keV$
C. $7.8 \; MeV$
D. $8.7 \; MeV$