2008 AMC12 A #17

Let $a_1 , \; a_2 , \; \cdots$ be a sequence of integers determined by the rule $a_n = \dfrac{a_{n-1}}{2}$ if $a_{n-1}$ is even and $a_n = 3a_{n-1} +1$ if $a_{n-1}$ is odd. For how many positive integers $a_1 \le 2008$ is it true that $a_1$ is less thatn each of $a_2 , \; a_3 ,$ and $a_4$ ?

(A) $250$          (B) $251$          (C) $501$          (D) $502$          (E) $1004$

정답 (D)