[单选题]
How fast should a moving cx*if(kwwe ;qr.2e qr3o5b;st v b 91fklock travel if it is tti 4-j7-vfe,ib dmb.no be observed by a stationary observer as running at one-half its normal rate,ei4f -vmt -b7djib. n?
A. $0.50 \; c$
B. $0.65 \; c$
C. $0.78 \; c$
D. $0.87 \; c$
