$$N$$ divisions on the main scale of a vernier calliper coincide with $$(N+1)$$ divisions of the vernier scale. If each division of main scale is $$a$$ units, then the least count of the instrument is

A

$$a$$

B

$$\frac{a}{N}$$

C

$$\frac{N}{N+1}\times a$$

D

$$\frac{a}{N+1}$$