[单选题]
A cyclotron of radius 0.25 m accelerates protons nh tm*lji7dh*f)v-as6 2c. fein a 1.7-T magnetic fin qo4,gjy/a9 ht1b4e held. What is th/qya,94 bj4 oeth1gh ne kinetic energy of the protons when they leave the cyclotron?
A. $7.8 \; keV$
B. $8.7 \; keV$
C. $7.8 \; MeV$
D. $8.7 \; MeV$
