Define a sequence $\{a_n\}_{n \geq 0}$ by $a_n = \sqrt{\frac{1+a_{n-1}}{2}}$ for $n \geq 1$,with $a_0 = \cos \theta \neq \pm 1$. Then,$\lim_{n \rightarrow \infty} 4^n(1-a_n)$ equals

  • A
    $\theta^2$
  • B
    $\frac{\theta^2}{2}$
  • C
    $\frac{\theta}{2}$
  • D
    $\theta$

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