[单选题]
A photon is a particl7wsh 4 mjkf,dkx 0 e--7n;qwrwwh; jg;w4/tdkd0 (od1izbarnj 2t.1gqhe69 ce that
A. has zero electric charge.
B. has zero electric field associated with it.
C. cannot travel in a vacuum.
D. has a velocity in a vacuum that varies with the photon frequency.