One main scale division of a vernier callipers is 'a' cm and n^{th} division of the vernier scale coincide with

(n – 1)^{th } division of the main scale. The least count of the callipers is mm is :

\(A) { 10na\over (n-1)}\)

\(B) { (n-1)\over (10n)}a\)

\(C) { 10a\over (n-1)}\)

\(D) { 10a\over n}\)