The periodic time of one oscillation for a simple pendulum is:

• A. \(2\pi \sqrt {\frac{g}{l}}\)
• B. \(\frac{1}{{2\pi }}\sqrt {\frac{g}{l}}\)
• C. \(2\pi \sqrt {\frac{l}{g}}\)
• D. \(\frac{1}{{2\pi }}\sqrt {\frac{l}{g}}\)