One main scale division of a vernier callipers is 'a' cm and nth division of the vernier scale coincide with
(n – 1)th division of the main scale. The least count of the callipers is mm is :
\(A) { 10na\over (n-1)}\)
\(B) { (n-1)\over (10n)}a\)
\(C) { 10a\over (n-1)}\)
\(D) { 10a\over n}\)