[单选题]
A cyclotron of radius 0.25 m accelerates protons *8o7(z0tal92)tvc)h xwxezjru er+ hin a 1. l9bk -ew,yoc s+ptm797-T magnetic field. What is the kinetic energy of the protons when they leave the cbp+sc yt-9l o79w km,eyclotron?
A. $7.8 \; keV$
B. $8.7 \; keV$
C. $7.8 \; MeV$
D. $8.7 \; MeV$
