A river gradually deepens, from a depth of $4 \ m$ to a depth of $8\  m$ as shown. The width, $W$, of the river does not change. At the depth of $4 \ m$, the river's speed is $12\  m/sec.$ Its  elocity at the $8\  m$ depth is ......... $m/sec$
  • A$12$
  • B$24$
  • C$6 $
  • D$8$
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