[单选题]
How fast should a movxh:+ylnw2r ,tgknwh 7r;gb 9*ing clock travel if ;;se.8bn qx dt-oc6e yit is to be observed by a stationary observer as running at one-half its nt-6x besnec d.o q8;;yormal rate?
A. $0.50 \; c$
B. $0.65 \; c$
C. $0.78 \; c$
D. $0.87 \; c$
