Let a1,a2,… be a sequence of integers determined by the rule a_n\equal{}a_{n\minus{}1}/2 if a_{n\minus{}1} is even and a_n\equal{}3a_{n\minus{}1}\plus{}1 if a_{n\minus{}1} is odd. For how many positive integers a1≤2008 is it true that a1 is less than each of a2, a3, and a4?
<spanclass=′latex−bold′>(A)</span>250<spanclass=′latex−bold′>(B)</span>251<spanclass=′latex−bold′>(C)</span>501<spanclass=′latex−bold′>(D)</span>502<spanclass=′latex−bold′>(E)</span>1004