A simple spring has length $l$ and force constant $k$. It is cut into two springs of length $l_{1}$ and $l_{2}$ such that $l_{1}=n l_{2}(n=$ an integer $) .$ The force constant of the spring of length $l_{2}$ is
(a) $k(1+n)$
(b) $\frac{(n+1) k}{n}$
(c) $k$
(d) $k /(n+1)$