Let $n$ be a natural number greater than or equal to 2.
A bag contains $n$ indistinguishable balls to the touch. All these balls have one ``HEADS'' side and one ``TAILS'' side except one which has two ``TAILS'' sides.
A ball is chosen at random from the bag and then tossed. The probability of obtaining the ``TAILS'' side is equal to: Answer A: $\frac { n - 1 } { n } \quad$ Answer B: $\frac { n + 1 } { 2 n } \quad$ Answer C: $\frac { 1 } { 2 } \quad$ Answer D: $\frac { n - 1 } { 2 n }$