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 spring of length $l_{1}$ is:
(a) $k(1+n)$
(b) $(k / n)(1+n)$
(c) $k$
(d) $k / n+1$