A coin is tossed twice. If events $A$ and $B$ are defined as :$A =$ head on first toss, $B = $ head on second toss. Then the probability of $A \cup B = $

  • A

    $\frac{1}{4}$

  • B

    $\frac{1}{2}$

  • C

    $\frac{1}{8}$

  • D

    $\frac{3}{4}$

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  • [AIEEE 2007]