[单选题]
A cyclotron of radius scd+ mb.aolh 19q )/*ymxg*su0.25 r :s 7kqx;g jy,9qn1wfm accelerates protons in a 1.7-T magnetic field. What is the kinetic energy ofj7q;x1 fs, :ngr9yk qw the protons when they leave the cyclotron?
A. $7.8 \; keV$
B. $8.7 \; keV$
C. $7.8 \; MeV$
D. $8.7 \; MeV$
