A biased coin with probability $p, \; 0 < p < 1$, of heads is tossed until a head appears for the first time. If the probability that the number of tosses required is even, is $2/5$, then $p$ equals :

Question

( A ) $1/3$
( B ) $3/5$
( C ) $2/3$
( D ) $2/5$