A weightless spring which has a force constant $k$ oscillates with frequency $n$ when a mass $m$ is suspended from it. The spring is cut into two equal halves and a mass $2 \mathrm{~m}$ is suspended from one part of spring. The frequency of oscillation will now become
(a) $n$
(b) $2 n$
(c) $\frac{n}{\sqrt{2}}$
(d) $n(2)^{1 / 2}$