[填空题]
Estimate (a) how long it took a stone to fal8d:d c0+*,qb bokcb2o:d +8nt:v gwdvrmp 9n al straight *(c th+-ypztk3 z*qas6 :tlcv down from the top of the Empire Stap k**ltqs-v+tyact3z6c( h:zte Building (420 m high) $s$, and (b) its velocity just before landing? $m/s$
