Given the function $f(x) = \frac{{{a^x} + {a^{ - x}}}}{2},\;(a > 2)$. Then $f(x + y) + f(x - y) = $

  • A

    $2f(x).f(y)$

  • B

    $f(x).f(y)$

  • C

    $\frac{{f(x)}}{{f(y)}}$

  • D

    None of these

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