, 2005

Nicholas is counting the sheep in a flock as they cross a road. The sheep begin to cross the road at $2{:}00$ p.m. and cross at a constant rate of three sheep per minute. After counting $42$ sheep, Nicholas falls asleep. He wakes up an hour and a half later, at which point exactly half of the total flock has crossed the road since $2{:}00$ p.m. How many sheep are there in the entire flock?

(A) $630$ (B) $621$ (C) $582$ (D) $624$ (E) $618$