[单选题]
How fast should a moving clock travel if it is to be xxt y2 c;0(*cu0rcv ayobserved by a stationa sd. 3,2ount*awzpsq;oly5h .ry observer as running d ,l;23 s.n.zoyhuota q5pw *sat one-half its normal rate?
A. $0.50 \; c$
B. $0.65 \; c$
C. $0.78 \; c$
D. $0.87 \; c$
