Suppose the probability that a coin toss shows "head" is $p$, where $0<p<1$. The coin is tossed repeatedly until the first "head" appears. The expected number of tosses required is

1. $p/\left ( 1-p \right )$
2. $\left ( 1-p \right )/p$
3. $1/p$
4. $1/p^{2}$
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