$A$ bead of mass $m$ is attached to the mid-point of a taut,weightless string of length $l$ and placed on a frictionless horizontal table. Under a small transverse displacement $x$,as shown in the figure,if the tension in the string is $T$,then the frequency of oscillation is:

• A
  $\frac{1}{2 \pi} \sqrt{\frac{2 T}{m l}}$
• B
  $\frac{1}{2 \pi} \sqrt{\frac{4 T}{m l}}$
• C
  $\frac{1}{2 \pi} \sqrt{\frac{4 T}{m}}$
• D
  $\frac{1}{2 \pi} \sqrt{\frac{2 T}{m}}$