One main scale division of a vernier callipers is a cm and nᵗʰ division of the vernier scale coincides with (n-1)ᵗʰ division division of the main scale. The least count of the calipers in mm is:
a. (n-1/10n)a
b. 10a/n
c. 10a/n-1
d. 1-na/n-1