[单选题]
How fast should a movingq. d. (a1cq*9jgiox0 gfc6wxy clock travel if it is to beoepqg+*g-u 6 blg.i mv:+vp-c xf8*jz observed by a stationary obse-z+vc-:ol.g +*gevi 8gfb ppxuq* jm6 rver as running at one-half its normal rate?
A. $0.50 \; c$
B. $0.65 \; c$
C. $0.78 \; c$
D. $0.87 \; c$
