A focus of an ellipse is at the origin. The directrix is the line $x = 4$ and the eccentricity is $ \frac{1}{2}$ . Then the length of the semi-major axis is

  • [AIEEE 2008]
  • A

    $\frac{8}{3}$

  • B

    $\frac{2}{3}$

  • C

    $\frac{4}{3}$

  • D

    $\frac{5}{3}$

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