[单选题]
A cyclotron of radius 0.25 t. 5 +r-ti i:ltlewe6cm acceln8 2h5x h/tfz/tqo-qgz4zrt /erates protons in a 1.7-T magnetic field. What is the kinetic energy of the proto-5qzz / /f4gqtxtz/rhhn ot 82ns when they leave the cyclotron?
A. $7.8 \; keV$
B. $8.7 \; keV$
C. $7.8 \; MeV$
D. $8.7 \; MeV$
