[单选题]
How fast should a moving clock travel if it is0ptvt /r:dxi0d i(0 0lew7eyvq2 wp6 f to be obs zdm5o)3+wqpn erved by a stationary observer as running at one-half its normal rapn5+zwm dqo 3)te?
A. $0.50 \; c$
B. $0.65 \; c$
C. $0.78 \; c$
D. $0.87 \; c$
