From the variation of potential energy in the direction of small oscillation of a simple pendulum, find the effective spring constant for the simple pendulum, where m is mass of the bob, \[l\] is length of the simple pendulum.

• A. \[\frac{mg}{l}\]            
• B. \[\frac{mg}{2l}\]
• C. \[\frac{2mg}{l}\]          
• D. \[\frac{mg}{\sqrt{2}l}\]