$13.$ A simple pendulum of length $\backslash ($ L $\backslash )$ and mass $\backslash ($ M $\backslash )$ has frequency $\backslash ($ f $\backslash ).$ To increase its frequency to $\backslash (2$ f $\backslash )$ :

A$)$ increase its length to $4$ L

B$)$ increase its length to $\backslash (2$ L $\backslash )$

C$)$ decrease its length to $\backslash (\backslash$mathrm$\{$L$\}/2\backslash )$

D$)$ decrease its length to L$/4$

E$)$ decrease its mass to $\backslash (<mi>\backslash </mi>$mathrm$\{$M$\}/4\backslash )$