If $0 < \theta, \phi < \frac{\pi}{2}$,$x = \sum_{n=0}^{\infty} \cos^{2n} \theta$,$y = \sum_{n=0}^{\infty} \sin^{2n} \phi$,and $z = \sum_{n=0}^{\infty} \cos^{2n} \theta \cdot \sin^{2n} \phi$,then:

  • A
    $xy - z = (x + y)z$
  • B
    $xy + yz + zx = z$
  • C
    $xyz = 4$
  • D
    $xy + z = (x + y)z$

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