[单选题]
A cyclotron of radius 0.25 m acceleratel xwhk3noo77cpsi y/m l,k9(.l *()rx;rd crns protons in a 1.7-T magnetic field. What is the kinetic energy of the protons whn x)r;c(l*r rden they leave the cyclotron?
A. $7.8 \; keV$
B. $8.7 \; keV$
C. $7.8 \; MeV$
D. $8.7 \; MeV$
